Sayo Aisaka is a ghost bound to Mahora Academy, having died roughly 60 years before the main timeline. Her appearance remains fixed at 15 years old, characterized by white hair, bright red eyes, and a height of 149 cm. She wears an outdated sailor-style school uniform from her era, distinct from modern Mahora attire. The manga typically depicts her without feet, while anime adaptations occasionally show her with feet. The circumstances of her death are ambiguous. Manga sources hint she may have been a victim of unsolved murders during the academy's founding, though she retains no memory of the events. The anime presents an alternate backstory: During World War II, after her mother's death, she lived with her younger sister. To fulfill her sister's wish to revive their mother, Sayo ventured into a storm to protect magical tsuwabuki flowers in the clock tower garden, subsequently succumbing to pneumonia. Initially invisible to almost all humans, Sayo spent decades isolated and unnoticed. Despite her longevity, her ghostly abilities—flight, intangibility, invisibility, object levitation, and possession—remained underdeveloped. She struggled with low self-esteem, often failing at basic spectral actions like scaring others and accidentally tripping despite lacking physical form. Profound loneliness fueled her desperate desire for connection. Faculty formally acknowledged her existence: Takamichi Takahata listed her as Student No. 1 in Class 2/3-A's roster and instructed new teacher Negi Springfield not to reassign her original seat. She remained overlooked until Negi inadvertently counted her during a class vote. Subsequent attempts to interact with classmates during Mahora Festival preparations caused misunderstandings, leading to near-exorcism by Mana Tatsumiya and Setsuna Sakurazaki. Negi and Kazumi Asakura intervened after photographing Sayo in distress, becoming her first friends. This acceptance marked a turning point; Evangeline A.K. McDowell, who had always sensed Sayo, later commended her newfound happiness. Post-acceptance, Sayo evolved from a timid spirit to a proactive ally. She formed a close bond with Kazumi, acting as her partner and assisting with camera operations during events like the "Mages vs. Mars" tournament. To leave Mahora's grounds—a limitation for earthbound spirits—she utilized a specialized doll provided by Kazumi, possessing it to interact physically and travel with the class. She accompanied Negi's group to the magical world, aiding search efforts for scattered classmates using Kazumi's surveillance gadgets; her camera alert prevented an attack on Nodoka Miyazaki by the "Black Hounds". In later developments, she contemplated forming a Pactio (magical contract) with Negi, questioning her capacity as a soul without a body while contrasting her state with artificial beings like Chachamaru. After contributing to the defeat of the Mage of the Beginning, she defied expectations by not passing on. Instead, she transitioned into a guardian spirit for Kazumi, cementing their partnership indefinitely.