Kotaro Sato, a four or five-year-old boy, lives independently in apartment #203 at the Shimizu Apartment complex. His remarkable self-sufficiency in daily tasks like shopping, cooking, cleaning, and laundry stems from severe parental neglect and abuse. His emotionally distant mother avoided physical contact, wearing gloves during baths or dressing, and openly neglected him, such as allowing him to get wet in the rain while she used an umbrella. His father, initially caring, later became physically abusive, declaring he no longer loved Kotaro. After his mother abandoned him and subsequently died, Kotaro entered a foster facility. He fled upon his father locating him, leading to his current solitary life under a restraining order against his father.

Financially, he receives weekly support delivered by lawyer Ayano Kobayashi; the money comes from his deceased mother's life insurance, though Kotaro remains unaware of her death or the payment's true source, occasionally expressing suspicion about its origins. Manifestations of his traumatic past include eating tissues due to starvation experiences, describing some brands as "sweet," and carrying a plastic toy sword while emulating his cartoon samurai hero, Tonosaman, adopting formal, archaic Japanese speech patterns.

Emotionally, Kotaro copes through symbolic acts and repressed vulnerability, like collecting four balloons, drawing faces, and labeling them "brother," "sister," "dad," and "mom" to create a temporary surrogate family, suggesting possible unmentioned siblings. He expresses two primary goals: becoming "strong" to regain his father's love and building a communal home for all his friends. Neighbors provide foundational support: Shin Karino, a manga artist evolving into a paternal figure, discovered Kotaro's mother's grave but withholds this truth; Mizuki Akitomo, a hostess acting as a maternal figure until fleeing domestic abuse; and Isamu Tamaru, a gruff neighbor often mistaken for yakuza, projecting paternal feelings onto Kotaro, though Kotaro consciously avoids becoming a replacement for Isamu's estranged son. These bonds form an ad-hoc family unit.

His development involves confronting past traumas, including a brief reunion with Ryota, a friend from the foster facility. When his father hires private investigator Aota to locate him, Kotaro deduces Aota's mission. Aota ultimately abandons the task after relating to Kotaro's abuse history, revealing his own childhood scars. Kotaro's journey emphasizes resilience, balancing childlike naivety with hard-won maturity, while his core desire for familial belonging persists.