Oshizu is the ghost of a girl named Murasame Shizu, who died in the old abandoned building of Sainan High School roughly four hundred years before the events of the story. She is usually called Oshizu, with the honorific O attached to her given name as a polite form of address, and she prefers this name. Because she was unable to move on, her spirit lingered in the old school building for centuries, unseen by the living, until her presence was discovered by Rito Yuki and his friends after rumors began to circulate about the site. Her encounter with them marks the beginning of her gradual entry into the modern world.

In personality, Oshizu is cheerful, sincere, kind, and energetic, with a curious nature and a slightly clumsy streak. She is fascinated by the modern world and eager to explore experiences that were closed off to her during her long years as a ghost. She is also notably absent-minded at times, and she carries a severe fear of dogs, a trait that parallels the fear of the supernatural felt by another character. Despite her gentle demeanor, she can become emotionally overwhelmed, and when frightened or startled she unconsciously releases bursts of mental stress that can blow apart objects and damage her surroundings. Related to this is a recurring tendency for these stress explosions to tear clothing, which frequently places those around her in embarrassing situations.

Oshizu possesses several supernatural abilities tied to her nature as a spirit. Along with her stress-induced psychic outbursts, she is capable of manipulating others through sheer willpower, and she can possess people. While possessing another person, she is able to sense that individual's emotions and feelings, which gives her insight into their private thoughts. This ability plays a significant part in one of her key roles in the story: after learning of Haruna Sairenji's romantic feelings for Rito, Oshizu becomes a supportive presence and actively encourages Haruna to confess her feelings to him. Her interventions are well meaning, though they do not always lead to the outcomes she hopes for.

Her role in the series shifts dramatically after the school doctor, Ryouko Mikado, creates a prosthetic body for her to inhabit. The body is designed with biological qualities, so it looks and feels like a real human body, effectively allowing Oshizu to return to the living. From that point she becomes a recurring character who attends Sainan High School alongside the main group and later works as a nurse and assistant in Ryouko's clinic, with Ryouko maintaining her physical body. Oshizu does not remain perfectly anchored to this body, however, and during brief moments of excitement her spirit can disconnect and leave the prosthetic form behind.

Her relationships with other characters are shaped by her warmth and helpfulness. She grows close to Rito and his circle of friends, and her friendship with Haruna is particularly meaningful because of her role as a confidante and supporter of Haruna's feelings. Her bond with Ryouko is also important, since Ryouko provides her with the means to live among the living and becomes her employer and caretaker. Through her experiences, Oshizu develops from a lonely spirit confined to an abandoned building into an active participant in daily life, gradually learning about friendship, romance, and the world she had missed for four centuries.