Sōsuke Mitsuba is a ghost connected to Kamome Academy, originally a middle school student and photography club member who died in a winter traffic accident. His ghost haunted the school's entrance lockers, visible only to those with supernatural senses, grabbing students' arms and demanding if they remembered him. In life, Mitsuba displayed a cocky, foul-mouthed personality, frequently boasting about his cute, androgynous looks despite being bullied for them in elementary school. To escape bullying in middle school, he adopted a passive, polite demeanor, resulting in classmates overlooking and forgetting him, deepening his loneliness. Beneath his arrogance, he showed genuine kindness, particularly towards Minamoto Kou, though he struggled to express it. His passion for photography was obsessive, inspired by his father's camera; he exclusively photographed valued subjects like nature and animals, winning several awards. He died rushing to buy potatoes to cook curry for his mother's birthday, revealing his unspoken care for her. After death, his ghost met Kou, a former classmate who hadn't noticed him while alive. Kou helped him fulfill a final wish to take a meaningful photograph, hoping to let him pass on peacefully. However, Yugi Tsukasa forcibly transformed Mitsuba into the "Broken Neck," a violent entity of eyes and camera parts bound by bandages, devoid of humanity and memory. Hanako destroyed this form to end its suffering. Tsukasa later recreated Mitsuba using a fragment of the original's soul and many lesser supernaturals, designating him the new Number 3 School Mystery after he consumed the previous Mystery's heart. This recreated Mitsuba, marked by a red scarf with tassels instead of the original's yellow scarf, retained similar mannerisms but lacked the original's memories and emotional ties. He became guardian of the Hell of Mirrors boundary, gaining powers to manifest in mirrors, manipulate reflections for information, and control his scarf as an extendable exoskeletal limb. Despite his role, he grapples with existential dread, fearing erasure and obsessing over becoming human to gain acceptance and friends. His relationship with Kou grew complex. Kou initially projected the original Mitsuba's memories onto the recreation, urging him to "remember." The recreated Mitsuba, aware he was a replacement, confronted Kou for ignoring his individuality, prompting Kou to acknowledge him as distinct. Kou then vowed to help him become human, a promise central to his purpose, though Mitsuba doubts its possibility. This dynamic intensified when Kou's unresolved guilt manifested a supernatural Mitsuba double in the Red House arc, reflecting Kou's subconscious desire for mutual dependence. Mitsuba's affiliation with Tsukasa's group complicated his autonomy, as Tsukasa manipulated his insecurities and powers. Mitsuba's core desires persist: longing for friendship, validation beyond appearance, and freedom from supernatural constraints. His favorite snack is flan pudding, he worries about his inability to build muscle, and a fan-designated birthday is March 28th, derived from the Japanese reading of "Mitsuba" (3-2-8).