Genta Kojima is a student in the first-grade class B at Teitan Elementary School. He lives with his parents, his father Genji Kojima and his mother, and is an only child who helps out at the family-owned liquor store called Kojima Liquors. Genta is physically the largest among his friends, with a strong build and a small circular bald spot on the left side of his head. His full name is derived from mystery writer Hajime Komine.

Genta is best known as the self-proclaimed leader of the Detective Boys, a group he helped form with his classmates after a case involving a supposedly haunted mansion. While he loudly declares himself the group's leader, his actual command abilities are often lacking, and both he and the other members instinctively look to Conan Edogawa for guidance during investigations. Despite his brash and outspoken demeanor, Genta possesses a strong sense of justice and is genuinely kind and dependable, especially when his friends are in danger.

A defining feature of Genta's personality is his massive appetite, particularly his love for braised eel on rice. His thoughts frequently revolve around food; when discussing large sums of money, he often measures their value in terms of how many bowls of eel he could buy. This focus on food often leads him to confuse similar-sounding Japanese terms, especially if one of them is food-related, and he sometimes struggles to read common kanji characters, which draws gentle teasing from his classmates.

His motivations are largely driven by a desire for adventure and his duty as a member of the Detective Boys. He uses his shoe locker at school as a mailbox for fellow students to submit cases for the group to solve. While he wants to be the leader, he is heavily reliant on Conan's superior intellect to solve the mysteries they encounter. His impulsive nature and lack of forethought, however, frequently land the Detective Boys in dangerous or sticky situations.

In terms of key relationships, Genta is a good friend to Mitsuhiko Tsuburaya and has a clear crush on Ayumi Yoshida, having known her since preschool. As a result, he often feels jealous of Conan, as Ayumi shows a clear preference for him. This rivalry, however, does not prevent Genta from trusting Conan's judgment when it matters. He has also shown some initial interest in Ai Haibara and sees her as a friend and fellow group member. Genta greatly admires his father, who he describes as a classic "Child of Edo". Across the series and in the feature films, Genta remains a consistent character who provides comic relief through his food-centric mindset and boasts, while also showing moments of unexpected bravery and steadfast loyalty to his friends. His physical strength is a notable ability, as he is considerably stronger than his classmates and is not afraid to use his size to intimidate or physically handle a situation.