In the anime film Maboroshi, Itsumi is a mysterious girl found living in captivity within the steel factory of the isolated town of Mifuse. Her background is central to the film's strange events. Itsumi did not originate from the frozen world of Mifuse but instead arrived there from the real world on a freight train. She was discovered as a small child by the fathers of the main characters. Rather than being returned, the town's leader, Mamoru Sagami, insisted she was a gift from the gods, and she was subsequently locked away in the fifth blast furnace of the factory for years to preserve the town's stagnant existence. Her real name is Saki Kikuiri, and she is the daughter of Masamune and Mutsumi from the real world, making her connection to the main characters deeply personal and familial.

Itsumi's personality is defined by the profound effects of her long-term isolation. When first encountered, she behaves like a wild animal, unable to speak or express her emotions properly. She has an unusual odor and does not understand normal human interaction, as she has been robbed of the experiences and mentorship needed for typical mental and emotional development. Despite her feral exterior, she is capable of forming bonds. She develops a significant connection to Masamune and Mutsumi, who become her caretakers, and her emotional world, though stunted, is intensely powerful.

Itsumi's primary motivation is not clearly stated by her but rather is inherent to her existence. She is the only being in the static town of Mifuse who physically grows and changes. Her intense emotions directly impact the fabricated reality of Mifuse, causing the cracks in the sky and other disturbances. The town's leadership thus works to keep her confined and emotionally subdued to prevent these events. For the other characters, the driving motivation becomes to save her. Masamune becomes determined to send Itsumi back to the real world where she belongs, a goal he ultimately achieves despite opposition.

Within the story, Itsumi serves as the linchpin for the entire conflict of Maboroshi. She is the physical manifestation of change in a world that fears it. Her presence is the reason the town remains in a state of suspended animation, as the leaders believe imprisoning her protects their reality. Her true identity as the future daughter of the teenage protagonists reveals that the frozen world is a constructed reality born from grief and loss, and she represents the hope and future that was taken away. Her escape from the pocket reality back to the real world is the climax of the film.

Itsumi's key relationships are with Masamune Kikuiri and Mutsumi Sagami. Masamune is the one who discovers her and gives her the name Itsumi. He becomes her protector and is the most dedicated to discovering her origins and setting her free. Mutsumi is initially her reluctant caretaker, keeping her distance to avoid emotional attachment. However, Mutsumi eventually gives in to her feelings, as the bond between them is later revealed to be a maternal one. In the real world, the teenage Masamune and Mutsumi are actually the future parents of Itsumi, making their connection in the frozen world a twisted reflection of their true familial relationship.

The character undergoes significant development, though it is largely defined by her circumstances. She begins as a nameless, imprisoned feral child and evolves into a girl who experiences love and a sense of belonging with her caretakers. Her ultimate development occurs when she is sent back to the real world. Though she departs unwillingly, heartbroken at leaving Masamune and Mutsumi, the film concludes with a glimpse of her years later as a young adult. She returns to the now-abandoned ruins of Mifuse, reflecting on her past as Itsumi and the place where her heart was first broken, indicating she has grown up and made peace with her traumatic experience.

Itsumi possesses notable and unique abilities that are central to the film's plot. Her primary power is her connection to the fabricated reality of Mifuse. Her emotions directly affect this world, causing the sky to crack and allowing glimpses of the real world to seep in. Unlike everyone else in the town, who are phantoms unable to age or change, Itsumi is a real person who continues to grow physically over the years she remains captive. This ability to mature, even while mentally stunted, makes her a dangerous anomaly that the town's leadership seeks to control. It is through her existence that the truth about the town and its inhabitants is ultimately revealed.