Jōji Shimaki, also known as George, is a veteran member of the Osaka Team who appears during the Nurarihyon mission. He has a distinctive appearance, being dark-skinned, bald, and muscular. In his civilian life, Shimaki works at a restaurant alongside his teammate and close friend, Nobuo Muroya, who is also known as Knob.

Shimaki possesses a personality typical of many experienced Osaka Team members: arrogant, aggressive, and dismissive of danger. He is highly confrontational, as shown when the Osaka Team first encounters the Tokyo Team, ordering them to leave and not steal any points. His arrogance extends to his comrades, as he and Muroya engage in competitive challenges to see who can kill powerful aliens faster. Despite appearing to be close friends with Muroya, when a battle turns against his partner, Shimaki refuses to help, demonstrating a careless attitude toward even his allies. Shimaki is described by a teammate as fighting recklessly, yet this aggressive approach has proven effective for him.

Shimaki has died and been transported to the Gantz room at some point in his past, and he has proven his worth by accumulating one hundred points on three separate occasions. Rather than choosing to be freed from the game, he has consistently decided to remain and continue fighting, indicating either a thrill-seeking nature or a lack of desire to return to his former life.

During the mission in Osaka, Shimaki hunts together with Muroya. The two engage in time-based competitions against powerful aliens, with Shimaki successfully killing one in what his partner calls just over a minute. He later confronts two of the mission's boss-level aliens, intervening to save members of the Tokyo Team in the process. After Muroya declares he will handle one alien alone and is mortally wounded, Shimaki steps forward to kill the other boss alien by himself. Using his proficiency with both the Z-Gun and katana, he dispatches this enemy, which is worth sixty-eight points, with relative ease.

When the final boss, Nurarihyon, arrives, Shimaki agrees to fight it together with Kazuo Kuwabara. The two slice the creature to pieces, but it immediately reforms in a different shape that has adapted to sword attacks. While Kuwabara retreats, believing the monster is too powerful and that another teammate named Oka will finish it, Shimaki refuses to give up. In a desperate final act, he grabs the alien, attempting to crush it with his bare hands. The plan fails disastrously when Nurarihyon transforms into a colossal mass of naked women, and Shimaki is killed, with only his neck remaining.

Shimaki is a capable fighter, proficient with the Z-Gun and katana, and his three cleared missions demonstrate his effectiveness despite his reckless style. His development in the story is limited, but he shows moments of determination and refusal to retreat even when faced with an enemy that has already overwhelmed his allies, ultimately leading to his demise.