Sunao Fujimori, a high school student central to the narrative, possesses distinctive pink hair and eyes. His hair is short in front yet extends to his waist in the back, typically secured by a red ribbon. His personality blends a quick temper with underlying vulnerability; he often reacts with physical aggression when provoked, especially towards his roommate Sora Hashiba. This surface irritability masks a caring nature, aligning with the "tsundere" archetype where initial hostility gradually reveals deeper concern and affection. A significant trait is his aquaphobia, an intense fear of water rooted in traumatic experiences, though this fear seems mitigated in non-canonical or later scenarios like hot spring visits.

His background involves childhood trauma: both he and Sora were abducted by a man named Aizawa and subjected to experiments designed to create alternate personalities. During this period, Sunao developed "Ran" as a secondary personality, influenced by Sora's own alternate persona, Yoru. Ran serves as a coping mechanism, exhibiting a more dependent and emotionally expressive demeanor, marked by red eyes and a higher-pitched voice. A pivotal escape attempt, orchestrated by allies Shinichirou Minato and Kai Nanami, ended with Sunao tripping and being left behind by Sora. This led to years of isolation under Aizawa's control, fostering a deep-seated desire for revenge.

Manipulated by Aizawa, Sunao returned to school initially feigning friendship to infiltrate Sora's life and execute revenge. Over time, he uncovered that Sora had been manipulated into abandoning him, shifting his focus from vengeance to reconciliation and protection. His relationship with Sora evolves from mutual distrust to romantic involvement, marked by pivotal moments like their first kiss and eventual physical consummation, symbolizing emotional resolution. The dynamic between their alternate personalities—Yoru and Ran—further complicates interactions, as these personas engage in intimate encounters without their hosts' consent, creating recurring tension and embarrassment.

Across media, Sunao's story includes key developments: in the visual novel "Rain," he disappears and is coerced into working for Aizawa; in "White Flower," he resurfaces years later, prompting a final confrontation with Aizawa that solidifies his reconciliation with Sora. The OVA depicts lighter moments, such as a group trip to a hot spring where his aquaphobia appears less severe, suggesting potential personal growth or non-canonical portrayal. His relationships extend beyond Sora, including mentorship roles with younger characters like Sei, Kitamura, and Shiina, who view him and Sora as familial figures by "White Flower's" conclusion. He also participates in school activities like the "Jack of All Trades" club, undertaking odd jobs that occasionally force him to confront personal fears or social challenges.