Ryuk

Ryuk is curious, mischievous, selfish, and fundamentally detached from human morality. Criminals, investigators, and Kira's supporters are all sources of entertainment to him rather than sides in a conflict he feels obliged to judge. When Light devises an elaborate plan, Ryuk is often less interested in whether it succeeds than in whether the process is entertaining.

He is unusually candid about that neutrality. Ryuk warns Light early that he should not expect friendship or protection simply because Ryuk owns the notebook that began everything. He sometimes withholds information until an especially inconvenient moment because he finds Light's reaction funny, and he generally makes Light reason through his own problems instead of supplying answers.

His great mundane weakness is apples from the human world. Ryuk compares them to cigarettes or alcohol for Shinigami and displays withdrawal-like behavior when deprived of them for too long. Light quickly realizes that apples can be used as an incentive, making them one of the few reliable ways of persuading Ryuk to perform minor favors.

As a Shinigami, Ryuk possesses a Death Note and can kill humans by writing their names while picturing their faces. He naturally sees the true names and remaining lifespans of humans without making the Shinigami Eye deal required of human notebook owners.

Ryuk is normally invisible and inaudible to humans who have not touched a Death Note associated with him. Physical contact with the notebook allows humans to perceive him, a rule that becomes important whenever Light must control who can see the Shinigami standing beside him.

He can fly, pass through ordinary physical obstacles, travel freely between the Shinigami and human realms, and endure conditions that would threaten human beings. His physiology differs radically from that of humans, and conventional human weapons do not function against him in any meaningful way.

The most consequential limitation on Shinigami behavior is that they exist by taking human lifespans. A Shinigami who deliberately kills a person in order to extend the lifespan of a human they care about dies, as demonstrated by Gelus and later by Rem. Ryuk never develops the kind of attachment that would place him in that position.

Ryuk creates the central situation of Death Note through boredom. He obtains an additional Death Note, writes instructions in English so a human can understand them, and drops it into the human world. Light finds it and begins testing whether the rules are genuine.

Once Light commits himself to becoming Kira, Ryuk follows him almost constantly. He explains rules when required, answers questions when he chooses, and watches Light's contest with L Lawliet unfold. Ryuk's presence also provides Light with information unavailable to an ordinary notebook owner, although Light can never assume that Ryuk will cooperate.

Ryuk periodically becomes directly useful. He accepts apples in exchange for small tasks, searches for surveillance devices because doing so benefits him, and interacts with other Shinigami when additional Death Notes enter the human world. None of those actions alter his stated principle that Light is responsible for his own choices.

When Light's final plan collapses and he can no longer escape the consequences of being exposed as Kira, Ryuk follows through on the warning he gave at their first meeting. Rather than wait for Light to spend an uncertain period imprisoned, Ryuk writes Light Yagami's name in his own Death Note. The conclusion reinforces that Ryuk was always an observer rather than Light's loyal companion.

Light Yagami is the human who finds Ryuk's Death Note and provides the entertainment Ryuk wanted when he entered the human world. Ryuk spends years observing Light Yagami but repeatedly states that he is not his friend and will not automatically help him.

Misa Amane becomes another human Death Note user within Ryuk's orbit. Ryuk is less personally involved with Misa Amane because her accompanying Shinigami is Rem.

Rem is another Shinigami active in the human world. Rem's intense protectiveness toward Misa contrasts sharply with Ryuk's refusal to become emotionally responsible for Light.

Gelus is a Shinigami whose actions demonstrate a rule Ryuk never intends to violate: Gelus dies after using his Death Note specifically to save Misa's life and thereby extending her lifespan.

The Shinigami King rules the Shinigami Realm. Ryuk acquires an extra Death Note before dropping one into the human world, setting the entire Kira case in motion.