Okinu, whose full name is Kinu Himuro, is a central figure in Ghost Sweeper Mikami. She is the ghost of an orphaned teenage girl who lived roughly three centuries before the events of the series. In her original life, she was sacrificed in a ritual intended to calm an angry volcano and prevent it from erupting. Ordinarily, a person who dies in this manner would become the guardian spirit of the mountain, but Okinu was not qualified to attain such a saintly or celestial state, so she lingered instead as an earth-bound spirit tied to the mountain and unable to find peace.

In terms of personality, Okinu is gentle, innocent, and somewhat timid, with a childlike quality that reflects her age at death. She is kind-hearted and compassionate, and she carries a deep loneliness from her long existence as a wandering spirit. Her motivations are rooted in her desire for connection and belonging, as well as her longing to be released from the sorrow of her unfinished life. She initially seeks to have a human sacrifice take her place as a way to escape her fate, but this desperate goal gives way to a more hopeful path once she encounters people who treat her with kindness.

Okinu's role in the story begins when Mikami Reiko and Tadao Yokoshima encounter her during their first major mission, a case involving a mountain hot spring haunted by two ghosts, one of whom is Okinu. When the mission concludes and she does not ascend, Mikami effectively adopts her into the Mikami Ghost Sweeper Company. From that point, Okinu becomes the third member of the team, serving as a secretary, errand girl, and general assistant. She also acts as a bridge between the living and the dead, offering a sympathetic perspective that contrasts with Mikami's more profit-driven and ruthless approach to exorcism.

Her key relationships are with the other two members of the company. She is devoted to Mikami Reiko, who gave her a place to belong, and she serves her with loyalty even though Mikami can be stern and demanding. With Tadao Yokoshima, Okinu develops genuine romantic feelings, making him the object of her affection. Yokoshima is often cowardly and easily frightened, especially by ghosts, but Okinu's warmth toward him is a recurring source of both comedy and tenderness. The lullaby known as Okinu no Komoriuta, a song her mother sang to her in life, is closely tied to her identity and appears as her theme throughout the anime.

Okinu undergoes significant development over the course of the series. Her arc moves from that of a restless, suffering spirit to that of a functioning member of a ghost-sweeping team, and eventually to a restored human life. In a major turn in the story, Okinu's spirit is reunited with her body, which had been safely preserved in ice over the centuries. She is revived and begins living as a normal teenage girl. During this new phase of her life, she temporarily loses her memories, but she gradually rediscovers who she is. In the process, she realizes that she is a natural necromancer with healing powers and a special ability to understand the pain of spirits. This makes her uniquely suited to her work and gives her a profound sense of purpose.

Regarding notable abilities, as a ghost Okinu is able to pass through walls, become invisible, or otherwise operate as a spiritual being, and her centuries of existence have made her deeply attuned to the supernatural world. After her revival, her abilities manifest as necromantic healing powers and an intuitive empathy with spirits, allowing her to sense and soothe their suffering. Her story embodies themes of grief, attachment, and the possibility of second chances, and she stands apart from the more menacing ghosts in the series as a figure of innocence and compassion.