Hitomi is the younger sister of Koichi Uehara and part of the Uehara family. A kindergartner often recognized by her pigtails, she initially greets Coo the kappa with intense hostility and resentment. She perceives his arrival as an intrusion disrupting her place within the family, a feeling crystallized when Coo uses her former baby chair at dinner, an act she sees as symbolic replacement. Her animosity surfaces through open rudeness and disdain directed at Coo.

A deliberate gesture of kindness from Coo breaks through her resistance, sparking a significant shift in her attitude. She gradually lowers her emotional defenses, developing a strong attachment and becoming fiercely protective of him. This evolution from hostility to deep affection defines her primary character development.

Despite the family's collective decision to keep Coo secret, Hitomi's young age makes this responsibility difficult. She inadvertently reveals Coo's existence to a school friend. This breach ignites widespread rumors, attracting intense media scrutiny that besieges the Uehara household and disrupts their lives. Her inability to maintain the secret, stemming directly from her youth, drives key consequences within the narrative.

Her journey illustrates a transformation from viewing Coo as a threat to embracing him as a cherished member of her family.