Seventeen-year-old high school student Yuichi Aizawa returns to a city he last visited seven years prior, staying with his aunt Akiko Minase and cousin Nayuki Minase. He arrives with significant memory loss regarding his childhood experiences, retaining only minor details. Forgotten memories gradually resurface through interactions with people from his past. He possesses a light complexion, hazel eyes, short brown hair, a slim build, and stands 173 cm tall. His typical attire includes a dark green jacket over a light beige turtleneck sweater or his school uniform: a blue jacket with a white sweater, dark grey tartan-patterned pants, and brown shoes. Yuichi maintains a friendly, easygoing, and down-to-earth personality, frequently engaging in playful teasing with female characters, varying in intensity but generally not malicious. He exhibits a selfless and patient nature, willing to go to great lengths to help others, especially girls connected to his past. Shiori Misaka observes a protective, almost older-brother-like quality in him. Despite his kindness, he displays occasional obliviousness to others' sensitivities. He fears heights and dislikes sweets. His return initiates interactions with five key heroines: Ayu Tsukimiya, a mysterious girl searching for a lost item; his cousin Nayuki Minase, who harbors long-standing feelings for him; Makoto Sawatari, an amnesiac girl holding a grudge; Shiori Misaka, a first-year student with a chronic illness; and Mai Kawasumi, a third-year student battling demons at their school nightly. Yuichi becomes intrinsically linked to each of their personal struggles. Across their stories, Yuichi provides unwavering support. He shelters Makoto despite initial reluctance and stays with her until her eventual death. His presence indirectly prevents Shiori from committing suicide. He aids Mai in fighting demons and uncovers her tragic past. With Nayuki, he recalls her childhood snow bunny confession and supports her during her mother’s hospitalization. For Ayu, he helps locate and repair a cherished lost item. Deepening involvement with each heroine leads Yuichi to encounter supernatural phenomena within the city, which he initially dismisses but gradually acknowledges. These experiences accelerate the recovery of his lost memories, revealing his own role in past traumas connected to the heroines. He fulfills both new promises and unkept childhood vows made to Ayu, Nayuki, Makoto, and Mai during his previous visit, rectifying them upon remembrance. In crossover contexts, Yuichi retains his core personality traits and relationships, interacting with characters from other franchises in comedic settings. His characteristic teasing, protective instincts, and supportive nature remain consistent, often referencing his established background like his relationship with Nayuki and past connection to the city. Yuichi's development centers on confronting his forgotten past and overcoming memory loss. As memories return, he acknowledges his unintended role in causing past pain and actively seeks redemption by fulfilling promises and providing emotional support. This journey transforms him from a detached, cynical individual into someone deeply connected to the city’s inhabitants and their struggles. His story concludes with him regaining most memories, resolving past traumas, and solidifying bonds with the heroines, particularly Ayu as his confirmed romantic partner in the anime adaptation.