Isao Hayama is the unemployed ex-gangster husband at the center of the story. His background is deeply rooted in the criminal underworld, having been a former Yakuza member, a fact symbolized by his missing pinkie finger, which is a traditional form of atonement for leaving the organization. Before the events of the narrative, he met his wife, Yukie Morita, during a period when she was living on the streets, struggling with drug addiction and working as a prostitute. He took a liking to her for reasons that remain ambiguous, and he ultimately saved her life by rushing her to the hospital after a suicide attempt, an act that created an unbreakable bond of obligation and love between them. Following her recovery, the two left his former life behind to start anew.

In his daily life, Isao is portrayed as boorish, lazy, and quick-tempered. He is unemployed and spends his days gambling, drinking, and generally getting into trouble while his wife works long hours at a noodle shop to support them. His most infamous behavior is his uncontrollable temper, which frequently manifests in violent outbursts, most notably flipping over the dinner table at the slightest provocation and sending their meal to the floor. Despite this aggressive and volatile exterior, he is not depicted as a one-dimensional villain. His character is rooted in frustration and a sense of obsolescence; he is a man who knows no other way of life besides that of a gangster and finds himself bored, frustrated, and deeply resentful of his mundane existence outside the Yakuza. His attempts to fit into mainstream Japanese life are often played for dark comedy, though his behavior is undeniably abusive.

Isao's primary motivation is not a clear goal or ambition but rather a complex mix of dependency, resentment, and an inability to adapt. He seems to have no desire to find legitimate employment or change his behavior. His role in the story is to serve as the primary obstacle to Yukie's happiness and the source of the central conflict. He is the "problem" that everyone around her advises her to leave. Yet, he is also the man who once saved her, creating the central emotional paradox of the narrative. His key relationships are almost entirely defined by his wife. He shares a profound, if dysfunctional, co-dependent bond with Yukie, whose love for him is unconditional precisely because of his past heroism. The story also hints at his connection to the criminal world through his former gangster status, though the details of his departure from it are a significant part of his backstory.

Isao does not undergo a dramatic personality overhaul in the traditional sense. The story's exploration of his character focuses more on revealing the history behind the man rather than showing him learn a new trade or become a model husband. His development is tied to the audience's and Yukie's growing understanding of the shared trauma and salvation that binds the couple together. He has no superhuman or specialized abilities. His notable traits are entirely behavioral and social: his skill for gambling, his capacity for sudden and violent outbursts, and his imposing presence as a former gangster. His physical strength and intimidating demeanor are the only tools he seems to possess, and he uses them to express his frustration within the confines of his small home and limited life.