Okinu, whose full name is Kinu Himuro, is one of the central characters of Ghost Sweeper Mikami. She is the ghost of an orphaned teenage girl who lived roughly three hundred years before the events of the story. During her lifetime she was chosen as a human sacrifice, dressed in a shrine maiden's outfit and offered to calm an erupting volcano. People who die in this manner usually become guardian spirits of the mountain, but Okinu was not qualified to become a celestial being or a saint, so instead she remained trapped as a land-bound spirit, unable to move on unless someone could take her place as a scapegoat. She was first met as one of two ghosts causing disturbance at a mountain hot spring resort, and after the incident was resolved she did not ascend to heaven. Because she could not pay for her own exorcism, she agreed to work at the office of the exorcist Reiko Mikami, becoming a secretary and errand girl for a small daily wage that was occasionally raised over the course of the series.

In personality, Okinu is gentle, warm hearted, cheerful, and exceptionally hardworking. She takes charge of cleaning and household duties in the office and sometimes goes to Tadao Yokoshima's home to cook for him. She is somewhat naive and openly admits that she is not particularly bright, but her strong dedication helps her perform her duties well and even earn good grades during the school lessons she attends. She is consistently considerate of others and rarely acts out of selfishness, which sets her apart from the more self-interested people around her. Because she experiences the world from the perspective of a ghost, she occasionally says things that seem strange or out of touch with ordinary human life, even though she is generally regarded as the most sensible member of the group.

Her main motivation is her desire to reach heaven and to find a place where she belongs. Working for the exorcist office gives her a purpose and a family-like group of companions, and she becomes deeply attached to them. As the story progresses, she develops romantic feelings for Yokoshima, who returns her affection, and the two are suggested to have shared a connection in a previous life. Her relationship with Mikami grows beyond that of an employee and employer into one of mutual trust and friendship, with Okinu acting as the emotional anchor of the office. She also forms meaningful ties with other characters, including the alchemist Doctor Chaos, who becomes connected to the circumstances of her possible revival, and Sanae Himuro, a shrine keeper's daughter who preserved Okinu's body and later welcomes her as family after she is restored to life.

Okinu's abilities come from her long existence as a spirit. Unlike most ghosts, she can interact physically with objects, make herself visible and audible to ordinary people who normally cannot perceive spirits, and pass through walls at will. She is able to control ectoplasm and can use her spectral nature in unexpected ways when the situation calls for it, though she can also be taken over by enemies who have the power to command ghosts. Having existed as a spirit for more than three centuries, she is an experienced presence in the supernatural world, even though her kindness means she rarely acts as a combatant. Her appearance reflects her past: she has long blue hair and wears the shrine maiden garments she was dressed in at the time of her sacrifice. Later in the narrative, after many trials, she is reunited with Mikami and Yokoshima and finally regains her full memories of the past three hundred years, including her time with them.