Shinpei Hidoro is the son of Tamotsu Hidoro, leader of the Hidoro crime family in the river port town of Barato. His mother, the family matriarch, subjected him to verbal and physical abuse, condemning him as "pathetic and ambition-less" like his father. Unknown to Tamotsu, his sterility meant Shinpei resulted from his mother's affair—a truth she disclosed during an argument, provoking Tamotsu to murder her in rage.

Shinpei embodies a "Boisterous Weakling," masking profound cowardice and terror of violence with loud bravado. He rejects his family's criminal legacy, aspiring only to escape it. His relationship with Chieko, his father's mistress, becomes pivotal; they fall in love, leading to her pregnancy. Shinpei plots to flee Barato with Chieko to forge a new life elsewhere, defying his heritage.

A confrontation erupts when Tamotsu discovers the affair and pregnancy. Tamotsu disowns Shinpei, branding him "useless" and expressing relief they share no blood. Shinpei retaliates with tearful defiance, swearing to surpass his father and offering him future employment if destitute. This unexpected mercy drives Tamotsu to attempt Shinpei's murder. Mid-attack, Hyakunosuke Ogata—an unconnected sniper—intervenes by shooting Tamotsu, moved by empathy upon recognizing parallels to his own familial trauma.

After Ogata denounces weakness, Shinpei escapes Barato with Chieko. His narrative concludes as they depart to prioritize their unborn child, abandoning criminal strife. His arc features in the "Barato" OAD bundled with the manga's 15th volume. He placed 101st with 128 votes in a 2018 popularity poll.