Jirō Amakusa is a character who appears in the story "The Lamb on the Chopping Block". He is a prideful boxer who trains at the Kanitani Gym. His initial encounter with the protagonist, Kōsaku, is accidental and humiliating; during a jogging session, he is accidentally punched in his left eye by Kōsaku, who was simultaneously doing his roadwork and delivering liquor. Overcome with shame, he covers his face and quickly runs off without saying a word. This event sets the stage for his role in the narrative, as it is the first time he has ever been knocked down, and the embarrassment drives his subsequent actions.

Jirō's personality is defined by his intense pride and an inability to cope with humiliation. He tracks Kōsaku to the Mukoda Gym, where he overhears his potential opponent's own career worries. Because Kōsaku was the first person to knock him down, Jirō arranges a match specifically to humiliate him in the ring as revenge. He lies to his own coach, claiming his black eye came from tripping, further demonstrating that his pride prevents him from admitting the truth. His pride is also coupled with a cold and calculating nature; during a pre-match interview, he claims his motivation is envy of Kōsaku's cheerfulness, as he himself cannot get over pathetic losses. When Sister Angela arrives, his attempt to punch Kōsaku backfires, resulting in his other eye being blackened and the moment being captured by a newspaper photographer, which only fuels his vow for vengeance.

During the match, Jirō's abilities as a competent boxer are showcased. In the first round, he is knocked down, but during the rest interval, he analyzes how Kōsaku could have such a poor record despite his clear power. In the second round, he uses his observational skills to notice that Kōsaku has weakened from eating before the match, so he lands a body blow intended to make him vomit. He easily batters the weakened Kōsaku in the third round, demonstrating his superior technical skill at that moment. However, his most notable moment of conflict comes from an unexpected source; he slips in a puddle of Kōsaku's vomit, which allows his opponent to land an uppercut that sends him out of the ring, resulting in a loss by a count of twenty.

His key relationship is, of course, with Kōsaku. He sees Kōsaku as both a source of unbearable shame and a perplexing rival. His development occurs after the match; despite his humiliation in the ring, he overhears Kōsaku outside, already over the loss and wishing to eat excessively again. At that moment, Jirō finally understands the reason behind Kōsaku's strange win-loss record, providing closure to his personal vendetta.