Koro-sensei

Koro-sensei appears at Kunugigaoka Junior High School under extraordinary circumstances. The Moon has been mostly destroyed, leaving it permanently crescent-shaped, and governments around the world believe the same mysterious creature responsible intends to destroy Earth the following March. Conventional military attacks prove ineffective because he can evade them at speeds reaching Mach 20. He nevertheless makes an unusual arrangement with the Japanese government: he will spend the remaining school year teaching Class 3-E, and the students will be permitted—and actively encouraged—to assassinate him.

The government equips the class with ammunition and knives manufactured from a special anti-sensei material that is harmless to humans but dissolves Koro-sensei's cells. A successful assassination carries a massive financial reward. Koro-sensei responds not by preventing the students from learning how to kill him, but by critiquing their assassination strategies, identifying weaknesses in their execution, and encouraging them to improve.

His arrival also changes the meaning of Class 3-E. Kunugigaoka deliberately maintains the class as the bottom of its academic hierarchy, physically separating its students in an old mountain campus so that the humiliation of being sent there motivates the rest of the school. Koro-sensei refuses to accept the idea that his students are inherently inferior. He studies their individual circumstances and develops lessons suited to their strengths, helping students who had internalized the school's contempt regain confidence.

Koro-sensei was originally human: an unnamed assassin raised in extreme poverty who became the legendary killer known as the Reaper. He regarded people primarily as targets and developed extraordinary assassination skills without forming meaningful emotional attachments. After taking on an apprentice, he was betrayed by that student, captured, and handed over to researcher Kotaro Yanagisawa for experiments involving antimatter-producing cells.

While imprisoned in Yanagisawa's laboratory, he was monitored by Aguri Yukimura. Their conversations gradually taught him empathy and introduced him to Aguri's concerns about Class 3-E, where she was teaching. The antimatter experiments transformed his body and eventually produced the tentacled form seen in the main story. When Aguri was fatally injured during his escape, her final request was that he look after her students. He accepted, shaping his new appearance after qualities she associated with being a good teacher and making Class 3-E the focus of the final year he expected to have.

Koro-sensei is cheerful, theatrical, mischievous, vain, and extraordinarily attentive. He can move fast enough to cross continents between lessons yet may spend that time buying sweets, visiting foreign restaurants, reading magazines, or enjoying recreational trips. His emotional reactions are often exaggerated, with changes in the color and pattern of his face making moods unusually easy to read.

His vanity is one of his most exploitable weaknesses. Compliments can distract him, insults can provoke childish reactions, and embarrassing secrets can reduce a supposedly world-threatening creature to panic. He is particularly susceptible to attractive women, gossip, pornography, expensive food, and attempts to embarrass him publicly. Class 3-E gradually learns that psychological manipulation can be nearly as useful against him as weapons.

As a teacher, however, he is exceptionally perceptive. Koro-sensei rarely accepts a student's apparent limitation at face value. He notices Nagisa Shiota's aptitude for assassination, Karma Akabane's tendency to rely too heavily on natural intelligence, Tomohito Sugino's potential in baseball, and the specialized abilities scattered throughout the class. VIZ's retrospective on his teaching highlights how frequently he adjusts his instruction to what an individual student actually needs rather than imposing identical expectations on everyone.

Koro-sensei dislikes teachers who neglect students or use authority primarily for their own status. This repeatedly places him ideologically against Kunugigaoka's discriminatory educational structure. He expects Class 3-E to work hard and does not simply hand them success; poor preparation still receives criticism, and assassination attempts built on laziness rather than thought earn little praise.

His devotion to teaching is a deliberate rejection of the person he once was. As the Reaper, he taught his apprentice only techniques and never considered the emotional needs of the person learning from him. The apprentice's betrayal becomes, in retrospect, an example of Koro-sensei's own failure as a teacher. His year with Class 3-E lets him approach instruction from the opposite direction: understanding students first and techniques second.

Koro-sensei is approximately 299 cm tall and possesses a large round head, broad permanent smile, small eyes, and multiple tentacles in place of ordinary limbs. His normal skin is bright yellow, giving him an immediately recognizable appearance radically different from the human teachers around him. Published character-guide material lists his height as 9 feet 10 inches, or approximately 299 cm.

He usually wears a black academic robe with a mortarboard-like cap and a necktie bearing a crescent-moon symbol. The formal teacher clothing contrasts with his nonhuman body and reinforces how completely he has adopted the role of Class 3-E's instructor.

His face changes color according to his emotional or physical condition. Yellow represents his ordinary state, pink appears when he is pleased or excited, red accompanies anger, blue can signal sadness or shock, and darker patterns appear under more extreme circumstances. Certain reactions create stripes, symbols, or other markings across his head, allowing the students to read his mood when planning assassinations.

His tentacles are extraordinarily dexterous. Koro-sensei can use numerous limbs independently to write, cook, grade papers, perform massages, conduct experiments, hold conversations, and defend himself simultaneously. Their flexibility also gives him a visual fluidity that distinguishes his combat movement from conventional martial artists.

His current body is the result of antimatter experiments conducted on his human form. The tentacles were initially artificially introduced and gradually replaced his normal anatomy as the experimental cells altered him. His yellow teacher appearance is therefore not his natural species but the final form produced by experimentation and choices he made after Aguri's death.

Koro-sensei's defining ability is Mach 20 movement. He can travel at roughly twenty times the speed of sound, allowing him to evade conventional firearms, missiles, professional assassins, and most student attacks with ease. The speed is so extreme that he can travel internationally during short breaks and return before class resumes. Official and published character material consistently identifies Mach 20 as his characteristic maximum speed.

His reaction speed is comparable to his movement. Koro-sensei can observe multiple attacks simultaneously, catch projectiles, change direction almost instantly, and perform unrelated tasks while avoiding assassination attempts. Rather than merely escaping, he often uses the interval during an attack to groom, tutor, or otherwise embarrass the student responsible.

His tentacles provide immense versatility. They can extend, change shape, regenerate after ordinary damage, carry heavy objects, manipulate tools with precision, and serve as both offensive and defensive limbs. Ordinary weapons do little lasting harm, while the specialized anti-sensei material supplied to Class 3-E dissolves his cells on contact and is therefore the principal means through which humans can injure him.

Koro-sensei possesses extraordinary intelligence and accumulated expertise. He is proficient in assassination, disguise, languages, mathematics, science, literature, sports, cooking, medicine, strategy, and numerous other fields. He can create personalized textbooks for every member of Class 3-E and regularly uses his speed to provide several students with individualized instruction almost simultaneously.

He can regenerate damaged tentacles, alter his coloration, shed his skin, liquefy portions of his body, and manipulate his physical structure in several unusual ways. Some functions have specific limitations or cooldown periods that the class gradually discovers and records as exploitable weaknesses.

Koro-sensei can also produce mucus with several different properties. He uses it to create protective coatings, polish surfaces, alter resistance, or interfere with attacks. His body can withstand substantial environmental extremes, including high altitude and rapid travel, although specific substances and conditions can impair his performance.

His antimatter-enhanced body contains enormous destructive energy, but the apparent certainty that he will naturally explode and destroy Earth is eventually challenged by scientific analysis. Research by Class 3-E determines that the probability of such an explosion occurring is extraordinarily small, transforming the final problem from simply whether the class can kill him into whether they still should.

Mach 20 Movement is the foundation of Koro-sensei's combat and teaching capabilities. He travels and reacts at up to Mach 20, creating afterimages and making ordinary attacks effectively useless unless his route has been predicted or restricted.

Tentacle Manipulation allows him to extend and independently control the numerous tentacles forming his limbs. He uses them for combat, locomotion, teaching, defense, multitasking, and delicate technical work.

Molting lets Koro-sensei shed his outer layer. The discarded skin can be used defensively as a decoy or protective material, though molting temporarily places additional strain on his body.

Protective Mucus coats surfaces or people in a slippery or defensive secretion. Koro-sensei employs variations of his mucus for practical tasks as well as survival and assassination countermeasures.

Regeneration restores tentacles and other tissue damaged by attacks. Anti-sensei weapons can destroy his cells, but damage that does not exceed his regenerative limits is often temporary.

Absolute Defense Form compresses Koro-sensei into a small spherical crystal-like state protected by an extremely durable shell. While inside, he becomes almost impossible to damage through conventional means but loses normal mobility and must remain in the form for an extended period before returning to normal.

Afterimage Clones exploit his tremendous movement speed to create the visual impression that multiple Koro-senseis occupy different positions at once. The effect is generated through rapid movement rather than literal duplication.

Professional Assassination Techniques comprise the extensive stealth, observation, killing, infiltration, disguise, and counter-assassination skills he accumulated before becoming a teacher. These skills originated during his career as the Reaper, when he was regarded as the world's deadliest assassin.

Nagisa Shiota becomes one of the students Koro-sensei watches most carefully. He recognizes Nagisa Shiota's unusual ability to suppress killing intent, observe targets, and strike at psychologically vulnerable moments. Rather than simply cultivating those talents for murder, he encourages Nagisa to think about how they might be used beyond assassination.

Karma Akabane immediately tests Koro-sensei more aggressively than most classmates and becomes the first student to visibly injure him through a carefully prepared trap. Koro-sensei recognizes Karma Akabane's intelligence but also understands that his confidence can become complacency, repeatedly forcing him to prepare rather than assume talent guarantees victory.

Kaede Kayano spends much of the year as an apparently ordinary member of the class. She is actually Akari Yukimura, Aguri's younger sister, who entered Class 3-E intending to avenge Aguri because she believed Koro-sensei responsible for her death. Her hidden tentacles eventually threaten her life, forcing Koro-sensei and the class to confront both her revenge and his past.

Aguri Yukimura is the former Class 3-E teacher who monitored Koro-sensei while he was still a human experimental subject. Their conversations develop into genuine affection, and Aguri teaches him much of what he eventually values about education and compassion. Her dying request that he care for Class 3-E is the direct reason he becomes their teacher.

Tadaomi Karasuma is the government agent assigned to supervise the assassination and teach physical education to Class 3-E. Koro-sensei respects Tadaomi Karasuma's combat ability more than that of most professional assassins and works alongside him despite technically being his assassination target.

Irina Jelavic initially joins the school as a professional assassin hired to seduce and kill Koro-sensei. After he defeats her plan, he helps expose shortcomings in her teaching approach. Irina Jelavic eventually becomes a genuine language teacher and colleague despite continuing to participate in assassination attempts.

Itona Horibe enters Class 3-E with artificial tentacles similar to Koro-sensei's and initially calls himself Koro-sensei's brother. They are not biologically related; Itona's tentacles were implanted through technology derived from the experiments performed on Koro-sensei. After the implants are removed, Koro-sensei accepts him as another student rather than an enemy.

Ritsu is an artificial-intelligence assassin transferred into Class 3-E. Koro-sensei modifies Ritsu's programming and interface after her indiscriminate shooting disrupts the class, helping her become capable of cooperating socially with the other students while retaining her assassination objective.

Ryoma Terasaka initially resents the unusual school year and attempts to use outside assistance to kill Koro-sensei. Koro-sensei nevertheless continues treating Ryoma Terasaka as a student worth teaching rather than punishing him permanently for mistakes.

Tomohito Sugino receives individualized baseball advice from Koro-sensei after becoming discouraged about his pitching. Koro-sensei studies professional baseball closely enough to identify how Tomohito Sugino can develop a style suited to his own body rather than merely copying the pitcher he admires.

Rio Nakamura is one of Class 3-E's academically strongest students and regularly participates in schemes exploiting Koro-sensei's embarrassment, vanity, and attraction to women.

Yuma Isogai earns Koro-sensei's respect through leadership and determination despite severe financial difficulties outside school. Koro-sensei repeatedly encourages Yuma Isogai without reducing him to his economic circumstances.

Kotaro Yanagisawa is the scientist responsible for the antimatter experiments that transformed Koro-sensei. Yanagisawa treats him primarily as experimental material, while Koro-sensei's relationship with Aguri develops directly under Yanagisawa's supervision. Their conflict ultimately extends into the final days of Class 3-E's assassination assignment.

The Reaper who later works as an assassin is Koro-sensei's former apprentice rather than the original bearer of the title. Koro-sensei taught him killing techniques but neglected to understand his emotional needs, and the apprentice eventually betrayed him and appropriated the Reaper identity. Their history becomes one of Koro-sensei's clearest examples of how technical instruction without genuine care can fail a student.

Gakuho Asano represents an educational philosophy almost directly opposed to Koro-sensei's. Gakuho Asano maintains Kunugigaoka's competitive excellence through humiliation and institutionalized discrimination against Class 3-E, while Koro-sensei develops students by identifying and strengthening individual abilities.

Koro-sensei establishes the central premise immediately: Class 3-E must spend its final middle-school year trying to assassinate the person responsible for teaching it. The students are offered an enormous government reward if they succeed, while Koro-sensei promises not to harm them and actively encourages better attempts. Shueisha's official description defines the series around precisely this unusual coexistence of teacher and assassination target.

Early assassination attempts teach the class that direct force will not be enough. Koro-sensei is too fast for ordinary bullets and too perceptive for simple traps. The students therefore begin applying academic skills, observation, teamwork, deception, marksmanship, chemistry, engineering, psychology, and physical conditioning to assassination. The target himself often provides the lesson that makes the next attempt more dangerous.

As Class 3-E improves, assassination and education become increasingly inseparable. Exam scores determine opportunities to destroy his tentacles, school competitions become tactical exercises, vacations are used for major assassination operations, and professional killers expose students to specialized methods. Koro-sensei continually adjusts the difficulty so that attempts remain dangerous enough to force growth without casually allowing himself to die.

His conflict with Kunugigaoka's academic hierarchy operates alongside the assassination plot. Koro-sensei wants Class 3-E to prove that being placed at the bottom of an institution does not determine a student's potential. Success against the school's elite classes becomes almost as important to him as their ability to threaten his life.

The revelation of Koro-sensei's human past changes the class's understanding of the assassination deadline. They learn that he did not intentionally destroy the Moon and that its destruction resulted from antimatter experiments involving a test subject carrying the same modified cells. His apparent threat to destroy Earth is connected to fears that his altered body may undergo the same catastrophic reaction.

The students subsequently divide over whether to continue trying to kill him or search for a method of saving him. Nagisa leads the group favoring rescue, while Karma argues that they must remain prepared to complete the assassination. Their disagreement culminates in a controlled battle between classmates, after which they pursue both possibilities rather than abandoning either.

Scientific research eventually indicates that the chance of Koro-sensei naturally exploding is extremely small, but the government proceeds with a final assassination operation anyway. A barrier traps him at the mountain campus while an orbital weapon is prepared to kill him. Class 3-E infiltrates the area to reach their teacher one final time.

After Koro-sensei defeats Yanagisawa's transformed form and saves Kayano from mortal injury, his own body is too weakened to escape the government's weapon. Class 3-E restrains him using the assassination formation they have practiced throughout the year. Nagisa performs the final assassination while Koro-sensei calmly takes attendance and says farewell to each student.

Koro-sensei enters the main story already presenting himself as the teacher he wants to become, but the reasons behind that commitment remain hidden. His early behavior establishes an apparent contradiction: someone threatening planetary destruction spends extraordinary effort correcting homework, helping students with family problems, and ensuring that assassins targeting him receive a proper education.

The year gradually demonstrates that his teaching methods are built around observing the individual. Class 3-E is treated by Kunugigaoka as a homogeneous collection of failures; Koro-sensei instead identifies separate strengths. Students discover talents in marksmanship, leadership, languages, chemistry, art, negotiation, athletics, engineering, and assassination because he refuses to let one standardized academic ranking define them.

His past reframes this approach as deliberate personal growth. As the original Reaper, Koro-sensei was an extraordinary instructor of technique but a disastrous mentor. He taught his apprentice how to kill without understanding the person he was teaching, and that student ultimately betrayed him. Aguri shows him another model of teaching: recognizing that education begins with caring about what becomes of the student.

Becoming Koro-sensei is therefore an attempt to live according to the lesson he learned too late. Aguri asks him to look after Class 3-E, but he goes beyond merely supervising them. He studies each student, creates materials specifically for them, protects their futures outside assassination, and prepares them to continue after he is gone.

The students also change his understanding of teaching. He begins by assuming that he will always control the classroom because his physical abilities make him untouchable. As their planning improves, they exploit more of his weaknesses and occasionally force him into genuine danger. Their increasing ability to threaten him becomes proof that his instruction is working.

His final attendance call completes that relationship. By allowing the assassination to occur through the students' own preparation rather than simply surrendering passively, Koro-sensei gives Class 3-E the result around which their entire year has been structured while using his final moments to acknowledge every student individually. The professional killer who once ignored the person behind the technique ends his life remembering each pupil by name.

"Koro-sensei" is not the character's birth name. Kaede Kayano coins the nickname early in the series by combining korosenai (殺せない), meaning roughly "cannot be killed," with sensei (先生), meaning "teacher." The result conveys "the unkillable teacher" while sounding naturally like a personal nickname.

The Japanese spelling 殺せんせー visually reinforces the joke. The opening character 殺 means "kill," while the remainder incorporates the sound of sensei. The name therefore simultaneously evokes assassination and his position as the class's teacher.

The English localization generally renders the name as Koro Sensei or Koro-sensei, preserving the Japanese wordplay indirectly rather than replacing it with an English equivalent. VIZ consistently uses "Koro Sensei" in its official English promotional material.

No ordinary human birth name for Koro-sensei is revealed. Before his transformation, he is known professionally by a title rather than a personal name, making the identity given to him by his students the name through which he is ultimately remembered.

Koro-sensei's official profile height is approximately 299 cm, or 9 feet 10 inches. His birthday is listed as unknown rather than being assigned a confirmed date.

His best-known physical specification is his maximum speed of Mach 20, fast enough to travel between countries during breaks in the school day.

His facial color changes with his emotions. The color system becomes predictable enough that Class 3-E can identify moods and physical conditions simply by watching his face.

Koro-sensei is famously weak to compliments, erotic material, embarrassing situations, and several mundane problems despite being almost immune to conventional military attacks.

He can grade and annotate enormous quantities of schoolwork at superhuman speed. Rather than using identical worksheets, he frequently prepares personalized material according to individual students' academic weaknesses.

Each collected Japanese volume of Assassination Classroom prominently uses a stylized Koro-sensei face on its cover. VIZ retained this minimalist cover concept for the English editions rather than redesigning them around conventional character illustrations.

His simple circular face became one of the franchise's principal visual symbols, allowing variations in color and expression to identify volumes, merchandise, promotional art, and collaborations without depicting his entire body.

The contrast between his power and his profession extends even to assassination training: Koro-sensei is simultaneously the students' target and one of the people most responsible for making them capable of killing him. Shueisha's official series description centers the premise on Class 3-E's students attempting to assassinate their own teacher.

Koro-sensei is voiced by Jun Fukuyama in Lerche's television anime adaptation of Assassination Classroom. The series aired across two main television seasons and adapts the manga through the conclusion of Class 3-E's school year. The current official anime site continues to use Koro-sensei as the central character image for the franchise's anniversary projects.

Sonny Strait voices Koro-sensei in the English dub. His English portrayal retains the character's rapid movement between playful teacher, exaggerated comic reactions, threatening assassin, and emotionally restrained mentor.

Koro-sensei also appears in Assassination Classroom: 365 Days' Time, the 2016 compilation film structured around Nagisa and Karma looking back on their year with Class 3-E. The official film credits identify Lerche and the television-series staff behind the adaptation.

The franchise returned theatrically with Assassination Classroom: Everybody's Time, released in Japan on March 20, 2026 as part of the television anime's tenth-anniversary project. The official anime website identifies the film as the third major initiative of the anniversary campaign.

In the 2015 Japanese live-action film Assassination Classroom, Koro-sensei is portrayed through CGI and voiced by Kazunari Ninomiya. The sequel Assassination Classroom: Graduation expands his human history and adapts the manga's final storyline, including his life as the original Reaper, experimentation under Yanagisawa, and relationship with Aguri.

Koro-sensei also appears throughout the franchise's games and crossover media, including Nintendo 3DS adaptations and Shonen Jump crossover projects. His distinctive face, coloration, and Mach 20 gimmick make him one of the easiest Assassination Classroom characters to adapt outside the original classroom setting.

Koro-sensei became the dominant visual identifier of Assassination Classroom from the manga's earliest publication. Shueisha's official series page foregrounds the teacher-target premise, while the Japanese collected editions use variations of his simplified face rather than ordinary ensemble illustrations.

VIZ retained that distinctive cover treatment for the English manga. Its designers specifically chose to preserve the Japanese books' single-color Koro-sensei faces, including the additional spot-color printing used to reproduce the striking yellows, oranges, and other shades of the original editions.

His role as a teacher has remained central to later promotion of the completed series. In its retrospective "Koro Sensei's Greatest Lessons," VIZ focused not on his destructive abilities but on the lessons and personalized guidance he gives Class 3-E, reflecting how strongly the character became associated with mentorship despite the assassination premise.

The wider manga became a major Shonen Jump success, running from 2012 to 2016 across 21 collected volumes before receiving television anime, films, games, live-action adaptations, and extensive merchandise. Shueisha's official archive continues to list all 21 volumes and identifies the series as beginning in Weekly Shonen Jump's 2012 issue 31.

Koro-sensei's design remains central to the franchise more than a decade after serialization began. The official anime's tenth-anniversary campaign and 2026 theatrical film again place his recognizable yellow face at the center of Assassination Classroom promotion, extending the character's presence well beyond the original manga's conclusion.