Tsukasa Yugi, the younger twin brother of Yugi Amane (later Hanako), was born on November 25, 1955. He attended Kamome Academy as a human student, wearing the old uniform: a white short-sleeved button-up shirt with black pants. As a child, he closely resembled Amane. They shared activities like watching artificial human movies and attending festivals, where Tsukasa persistently played katanuki paper-cutting games for prizes, often skipping meals. When Amane's health mysteriously declined, the four-year-old Tsukasa entered the supernatural Red House. He sacrificed himself to an entity inside to wish for Amane's recovery, disappearing for six months. Upon returning, Kunishige noted Tsukasa's drastically changed behavior, displaying supernatural knowledge and abilities; his mother believed him possessed. Priests found no evidence, though Tsukasa accurately predicted a shrine head priest's death and revealed Kunishige's unspoken wish for it. Tsukasa adopted a cheerful yet unsettling demeanor after his return. He believed Amane hated him, a perception solidified when Amane pushed him away physically. This belief deepened after Kou Minamoto and Nene Yashiro, traveling back in time, inadvertently revealed Amane's future act of killing him. Trapped in the Red House, Tsukasa initially seemed innocent but expressed excitement upon learning of his impending death. He chose to stay in the burning house with the entity, declaring they would "return together." They formed a symbiotic relationship; the entity resided within him without overriding his consciousness, allowing Tsukasa full agency as they collaborated. Decades later, as a ghost haunting Kamome Academy, Tsukasa appears identical to Amane but possesses constricted pupils, visible fangs, and a black seal on his right cheek marked with the kanji "封" (seal), signifying his role as Amane's yorishiro. He wears a modified uniform: a white Western shirt, black kimono, grey hakama, black shoes, and red ankle socks, and commands two black koku-joudai spirit orbs with pink and blue markings. Residing in the Broadcasting Club room with Nanamine Sakura and Hyuuga Natsuhiko, Tsukasa grants wishes for the deceased. His personality blends extreme childishness, including overt affection for Amane, with remorseless violence—exemplified when he dissected a pufferfish to "see its insides." He manipulates others to achieve goals, like altering rumors to transform Mitsuba Sousuke's spirit into the monstrous "Bent-Neck Boy at the School Entrance." He enforces compliance through intimidation, refusing to back down unless obeyed, as during the Hell of Mirrors incident. He frequently references his own murder, claiming Amane's expression during the act was his "favorite." His motivations include disrupting the balance between humans and supernaturals, believing both should act unrestrained, opposing Hanako's efforts to maintain order. Tsukasa commands koku-joudai for illusions and combat, possesses enhanced combat prowess enabling him to defeat powerful supernaturals like the former Number 3 in their own domain, and can induce sleep via touch. Persistent themes define his background: he prioritizes Amane above all, listing him as his "favorite snack," "latest concern," and "preferred type." Though Amane concluded Tsukasa was an "imposter" due to his changed behavior and supernatural traits, evidence supports Tsukasa retaining his core identity and memories, including childhood selflessness and insecurities about Amane's hatred. His development shows continuity from his human willingness to sacrifice himself for Amane to his ghostly actions driven by a desire to understand and provoke emotional honesty from his brother. In the new timeline created by the Clock Keepers, Amane Yugi exists as a human teacher at Kamome Academy, implying Tsukasa's fate may differ.