Sosuke Mitsuba exists in multiple iterations across distinct timelines. Originally a human student at Kamome Academy's Middle School Division and member of the Photography Club, he perished in a winter road accident while purchasing potatoes to cook curry for his mother's birthday. Posthumously, he manifested as a ghost haunting school lockers, visible solely to supernatural affiliates. This spectral form would grasp people's hands while questioning if they remembered him, echoing his profound loneliness and craving for recognition. Yugi Tsukasa forcibly transformed this ghost into the "Broken Neck" supernatural—an entity stripped of humanity, composed of eyes and camera parts bound by bandages. It attacked indiscriminately until Yugi Amane terminated its existence to end its suffering. Tsukasa later recreated a new Mitsuba using a fragment of the original's soul and component supernaturals. This iteration consumed the heart of the former School Mystery No. 3, ascending as the new representative of the "Hell of Mirrors" and gaining dominion over its boundary. His appearance evolved dramatically, now featuring a red scarf with tassels, a tattered black jacket adorned with eyes, bird-like talons, and feathers threaded through his hair. Mitsuba exhibits arrogance, narcissism, and a foul mouth, frequently bragging about his looks and insulting others—traits rooted in childhood bullying over his girlish appearance. This led him to adopt a passive, polite facade in middle school, which backfired, rendering him forgettable to classmates. Beneath his abrasive exterior, he harbors deep-seated loneliness, vulnerability, and sporadic kindness toward trusted individuals. His photography passion borders on obsession; though he won multiple awards, he focused exclusively on nature and animals, avoiding people. He used his father's camera, and his talent was innate. In the "New Present" timeline—altered by the Clock Keepers—Mitsuba never died. Alive and attending Kamome Academy's Middle School Division, he shares a close friendship with Minamoto Kou but retains no memory of Yashiro Nene. He displays timidity toward the supernatural yet demonstrates bravery when motivated, such as searching for a missing underclassman. He often teases Kou, particularly about interactions with girls. This version experiences dreams of his original self's death and the fake iteration's transformation into No. 3, fueled by a mounting curiosity to reclaim these memories. Both deceased versions manifest in these dreams, urging him to remember. As the current No. 3 School Mystery, he wields powers tied to the Hell of Mirrors boundary: manifesting through mirrors, extracting information from reflections, and manipulating his scarf's exoskeleton-like extensions as limbs. These appendages may attack involuntarily when threatened. In the New Present, he lacks supernatural abilities but exhibits partial memory retention akin to characters like Kou and Aoi. Trivia includes Japanese fans celebrating "Mitsuba Day" on March 28 (a play on his name's pronunciation), his human fear of lightning, and his preference for flan pudding.