Kosuke Nakajima is a minor character in Gantz. He is a twenty-eight-year-old member of a Bosozoku motorcycle gang led by Tetsuo Nemoto, and is summoned into the Gantz apartment along with the rest of his crew for the mission to eliminate the Tanaka Aliens. Nakajima is of average height and weight, with short grey-blue hair and a thuggish appearance.

In the Gantz room, the gang immediately attempts to assert dominance over the other participants and claim the available weapons. When the veteran player Joichiro Nishi refuses to cooperate, Nemoto orders his men to kill him. Acting as Nemoto’s subordinate, Nakajima picks up an X-gun and fires at Nishi. Nakajima is driven by a desire to intimidate others and a blind loyalty to his gang leader. He possesses no special abilities or combat training.

Before the mission officially begins, Nakajima’s attack fails. Nishi’s Gantz suit absorbs the energy blast, leaving Nishi entirely unharmed. Without hesitation, Nishi retaliates by seizing the weapon and killing Nakajima with it. Kosuke Nakajima is the first casualty of the Tanaka Alien mission, dying within the Gantz room itself. He experiences no development, as his role in the story is limited to this brief but violent confrontation that demonstrates the immediate peril of the Gantz game and the ruthless nature of Joichiro Nishi.