Mitsuko Kawai is a young girl who lived in a rundown apartment building with parents described as extremely neglectful. Her mother frequently failed to pick her up from school, leaving the responsibility to her father, and the household provided little supervision or care. One day, while exploring the rooftop of the building, Mitsuko attempted to retrieve her red schoolbag and accidentally fell into one of the large water tanks. She drowned inside the tank, and her body was never recovered, causing her to be listed as a missing person rather than a confirmed deceased.

After her death, Mitsuko's spirit remained trapped within the building, manifesting as an onryo, a vengeful Japanese ghost. Her primary motivation is a deep, unresolved longing for maternal love and attention, which she never received in life. This unmet need drives her to seek a mother figure, and she fixates on Yoshimi Matsubara, a divorced mother who moves into the apartment with her young daughter, Ikuko. Mitsuko's haunting is initially expressed through small disturbances, such as the reappearance of a red schoolbag and the sound of footsteps from the abandoned unit above, but it escalates into direct attempts to harm the living.

In the story, Mitsuko serves as the central antagonist, though she is portrayed more as a tragic figure than a malevolent force. Her role is to force Yoshimi to confront the themes of motherhood, sacrifice, and loss. Mitsuko's key relationship is with Yoshimi, whom she ultimately claims as a mother figure. She also targets Ikuko, intending to take her place as Yoshimi's child. Over the course of the narrative, Mitsuko develops from a haunting presence into a desperate spirit who will not release her hold until she receives the care she craves. The resolution comes when Yoshimi voluntarily sacrifices herself, embracing Mitsuko and becoming her mother in the afterlife, which finally grants the ghost the love and attention she sought.

As a supernatural entity, Mitsuko exhibits abilities typical of an onryo. She can manipulate water, both as a persistent leak and as a drowning hazard, and can control the environment around her through telekinesis. She also possesses the ability to change her form, appearing at times as a decaying corpse and at others as a normal young girl, to confuse or terrify her victims. Despite her formidable powers, her actions are driven not by malice but by a childlike need for affection, making her a sorrowful rather than purely fearsome character.