Keiichiro Miyanoshita is a main character and the youngest member of the group confronting supernatural events. He is a first-grade elementary school student, approximately six to seven years old, and the younger brother of Satsuki Miyanoshita. Physically, he has fair skin, short purple-gray hair, and brown eyes. His typical outfit includes a light blue shirt with an orange-red rectangle and the letter "K," paired with slate-gray shorts and yellow calf-high boots.

Keiichiro is defined by a shy, sweet, and deeply lonely nature. He is very close to his older sister Satsuki and his father, though the family dynamic remains strained following the death of his mother, Kayako Miyanoshita, who passed away when he was in kindergarten. This loss is central to his character; he deeply misses his mother and carries a profound fear of losing more family members. This fear makes him easily frightened and prone to crying, leading to him often being described as a "crybaby". He is portrayed as naive, immature, and somewhat clumsy, and his vulnerability often places him in danger or creates situations that the other characters must resolve.

Despite his timid disposition, Keiichiro’s role in the story is significant. His innocence acts as a catalyst for the protective instincts of the older characters, particularly his sister. He forms a unique and special friendship with the demon Amanojaku, who possesses the family cat, Kaya. While the other characters address the demon by his true name, Keiichiro consistently calls him "Kaya," a sign of his compassionate and accepting nature that helps forge their unlikely bond. He looks up to his sister's friends, Hajime Aoyama and Leo Kakinoki, viewing them as surrogate older brothers, and he enjoys playing with Momoko Koigakubo. His natural kindness even extends to other spirits, as he demonstrates a peculiar talent for befriending them unintentionally.

Throughout the series, Keiichiro undergoes notable emotional development. While he begins as a fragile child weighed down by grief, he gradually learns to accept his mother's death and displays rare but significant moments of courage when confronting the ghosts that threaten his loved ones. In terms of abilities, he does not possess strong spiritual powers like his mother. However, his innocence and emotional sincerity allow him to connect with spirits in a way others cannot, often helping to resolve conflicts through empathy rather than exorcism. The legacy of his mother, a powerful psychic who authored a ghost diary used to fight spirits, hangs over him and his sister, though Keiichiro himself does not inherit her abilities.

A significant distinction exists between the original Japanese portrayal of Keiichiro and the English-language adaptation produced by ADV Films. In the original version, he is a neurotypical child whose immaturity and emotional fragility are rooted in genuine grief and his young age. In the ADV English dub, however, his character is reinterpreted with traits suggesting a mental disability, including references to dyslexia, nonsensical or babbling speech patterns under stress, and dialogue that includes inappropriate or darkly humorous comments about his own limitations. This adaptation shifts the focus away from his emotional growth and towards exaggerated comedic effect, creating a markedly different version of the character.