Isamu Tamaru, a divorced father estranged from his son after his ex-wife severed contact despite the boy’s wish to reconnect, channels his paternal yearning into an attachment to Kotaro, a four-year-old neighbor he treats as a surrogate child. His boisterous, abrasive exterior masks vulnerability, often flaring into jealousy when others like Shin Karino grow close to Kotaro. Though he relentlessly seeks to dote on the boy, Kotaro deflects his overeager affection with a toy sword, enforcing playful boundaries.

Broad-shouldered and rugged, Tamaru sports a scarred face and a style evoking yakuza stereotypes, inviting assumptions of criminality that draw police attention during his outings with Kotaro. Yet beneath his intimidating guise lies a fierce protectiveness toward children, particularly Kotaro, whom he shields with unwavering dedication.

Though his profession is unspecified, rumors of underworld ties linger. His fractured history with his biological son includes a restraining order and a pivotal confrontation when the boy once defied his mother to seek Tamaru—a moment that starkly reveals his struggle to balance legal restraints with paternal longing.

Initially portrayed as a comically forceful figure, Tamaru gradually unveils layers of sincerity and devotion, prioritizing Kotaro’s safety above all. His arc weaves themes of redemption and found family, as he tentatively navigates his unresolved role as a father while bolstering Kotaro’s circle of support.