Chiaki Onizuka

Onizuka is aggressive, predatory and confident in his physical size. He casually threatens sexual violence after hearing other delinquents discuss Katou and assumes that his reputation and build will allow him to dominate the younger student.

His behavior toward younger students establishes him as a bully rather than someone interested in a fair contest. He is comfortable occupying their restroom and intimidating people who are physically less imposing.

That confidence collapses when confronted by someone stronger than expected.

Onizuka is an ordinary human with no supernatural abilities and no connection to Gantz.

He is notably large and muscular for a high-school student, giving him an intimidating physical presence. The story does not establish formal martial-arts training or other specialized combat skills.

His size ultimately proves insufficient against Katou, whose physical strength and anger overwhelm him in their confrontation.

Onizuka appears during the portion of the story exploring Katou's ordinary school life between missions. After overhearing younger delinquents discussing Katou, he makes a sexual threat against him.

Katou learns about the threat and confronts Onizuka directly in the restroom. He beats the older student severely and then warns the surrounding delinquents to stop bullying weaker people.

The incident demonstrates an important boundary in Katou's pacifism: although he hates killing, he is capable of decisive violence when confronting someone who deliberately victimizes others.

Masaru Katou becomes Onizuka's target after he hears other students discussing him. Their confrontation ends with Katou overpowering him and rejecting the intimidation surrounding his reputation.

Kazuki Shiro learns about Onizuka's threat and warns Katou about what he intends to do.