Jirōmaru is a supporting character in the 2015 anime film The Boy and the Beast, where he serves as the younger son of Iouzan, a respected and powerful elder in the beast realm. As a child, Jirōmaru possesses a worldview shaped entirely by physical strength, believing that worth is measured by one's ability to fight and dominate others. This belief leads him to initially bully and attempt to beat up the human boy Kyuta, whom he views as weak and inferior simply for being human. However, Jirōmaru's perspective undergoes an immediate and significant shift when Kyuta manages to defeat him in a minor scuffle. Rather than harboring resentment, Jirōmaru is genuinely impressed by Kyuta's strength and quickly warms to him, transforming from a bully into a friend.

Physically, Jirōmaru is recognizable by his thick, brown unibrow, a trait that persists from childhood into his teenage years. As an adolescent, he sports a pomp hairstyle and has brown hair and brown eyes. By this point in the story, his personality has mellowed considerably, and he emerges as a friendly, kind, loyal, and strong individual. His relationship with his older brother, Ichirōhiko, is complex and evolves significantly over time. Jirōmaru greatly admires his brother's strength during their childhood, with Ichirōhiko positioned as the responsible older sibling who chastises Jirōmaru for his bullying behavior. Following a time skip, the dynamic between the brothers reverses. Jirōmaru matures into someone who shows equal respect to both his father, Iouzan, and Kumatetsu, Kyuta's master and father figure. By contrast, Ichirōhiko becomes more bitter and arrogant, haughtily dismissing both Kumatetsu and Kyuta as inferior. Even when Jirōmaru witnesses his brother's corruption and learns the truth of Ichirōhiko's adoption and human origins, it changes nothing between them. Jirōmaru remains steadfast in his loyalty, considering Ichirōhiko his brother regardless of what anyone else thinks.

Jirōmaru's role in the story primarily centers on his relationships with Kyuta and Ichirōhiko. His early antagonism toward Kyuta establishes some of the initial conflicts Kyuta faces in the beast realm, while their subsequent friendship demonstrates Kyuta's growing acceptance and capability within that world. Jirōmaru's unwavering acceptance of Ichirōhiko despite his brother's corruption highlights themes of familial loyalty and unconditional love. Regarding notable abilities, no specific combat techniques or powers are detailed as unique to Jirōmaru. He is described as strong and capable as a fighter, consistent with his upbringing in the beast realm's martial culture, but no signature attacks or special abilities are documented.