Oshizu is the affectionate nickname of Murasame Shizu, a ghost girl who died roughly four hundred years before the events of the story and whose spirit long remained in the old abandoned school building at Sainan High School. Her formal given name is Shizu, but she prefers to be called Oshizu, the honorific form that the other characters use for her. Because she spent centuries isolated in the decaying structure, she is curious, energetic, and fascinated by the modern world, while remaining notably clumsy and somewhat absent-minded. She is sincere, gentle, and kindhearted by nature, though she carries a strong fear of dogs that mirrors the fear other characters have of the supernatural. After meeting Rito Yuki and his friends, she begins to venture out of the old school building and gradually becomes part of the wider group.

Her motivation centers on experiencing the world she missed for so long and on connecting with the people around her, particularly in matters of the heart. Through her special ability to possess other people and explore their subconscious minds, she can read their memories and sense their true feelings toward others. It is through this ability that she becomes aware of Haruna Sairenji's long-held crush on Rito, and she grows very supportive of the idea of helping Haruna finally confess her feelings to him. Oshizu acts as a gentle encourager in that romance, using her unusual insight into emotions to nudge the relationship forward.

In terms of her role in the story, Oshizu functions as a recurring supporting presence who brings supernatural comedy and heartfelt moments to the cast. Her ghostly nature supplies many of her defining scenes, and in the To Love Ru: Darkness portion of the story her situation changes significantly when the doctor Ryouko Mikado creates a prosthetic body for her with realistic biological qualities, effectively allowing Oshizu to return to a physical life. She then works as a nurse and assistant in Ryouko's clinic, though her spirit still occasionally slips out of the body during moments of excitement, leaving the empty form behind. In the earlier anime adaptation she is not given this new body and her role is more limited, whereas the OVA releases and the Darkness series follow the version in which she has her prosthetic form. She also plays a meaningful part in a major crisis of the Darkness storyline by helping separate the characters Mea and Nemesis, which allows Mea to pursue her own freedom and contributes to the resolution of the conflict involving the assassin known as Golden Darkness.

Her notable abilities are rooted in her identity as a powerful ghost. She can possess other people and control their movements, and while possessing them she can enter their subconscious to examine their memories and understand their feelings, an ability that works somewhat like psychic diving. When she is frightened, she can release blasts of psychic stress strong enough to blow apart objects and the surrounding area, and she can also manipulate others through sheer force of will. A recurring comic side effect of these stress blasts is that they frequently tear clothing apart, placing Rito in awkward situations with the female characters.

Her key relationships anchor her development across the series. Ryouko Mikado is the figure most responsible for her new lease on life, building the prosthetic body that lets her live among the living and serving as her employer afterward. Haruna is one of her closest friends, and Oshizu's support of Haruna's romantic feelings shows her compassionate side. Rito is the person who first draws her out of her isolated existence, and her friendship with him and the wider group gives her a sense of belonging she lacked for centuries. Her real name, Murasame Shizu, is revealed over the course of the story, and she confirms her true identity during a moment in which she helps prevent a catastrophe, showing how far she has come from the lonely ghost of the old schoolhouse.