Keisuke Saji is a 16-year-old high school student with spiky hair. Initially portrayed as a typical hormone-driven teenager fixated on women and sexual fantasies, his personality transformed significantly during childhood. Originally shy and reserved, missing the chance to confess to his first love due to illness sparked a transformation, leading him to adopt a "no regrets" approach to expressing his feelings. This manifests as a relentless, undeterred pursuit of his classmate Akira Koizumi, despite her violent rejections and physical assaults.

His defining traits include shameless perversion and persistent romantic dedication. After stealing his father's cell phone and receiving a suggestive call from "Alice," he discovers Koizumi orchestrated the prank targeting older men. Their subsequent encounter leads to a date where he gifts her a ring, followed by a sexual encounter where Koizumi loses her virginity to him. Discovering bloodstained sheets confirming her virginity, he becomes devoted to her, dismissing rumors about her alleged promiscuity or lesbianism. His commitment persists even as she publicly denies their relationship and physically punishes his constant advances.

While single-mindedly focused on Koizumi, he frequently engages in lecherous behavior toward other women, including fantasizing about classmates, stealing underwear, and attempting opportunistic groping. He often internally apologizes to Koizumi during these acts but rarely curbs his impulses. His childhood friend Yuki Umemiya, who harbors unrequited feelings for him, becomes an unintentional rival; he rejects her repeated seduction attempts despite her physical attractiveness. He also attracts unwanted attention from Sakurai, a gay underclassman with a crush on him, which he awkwardly deflects.

His loyalty to Koizumi deepens as he protects her from dangerous situations, including bullying and attempted rape. His straightforwardness and refusal to hide his desires gradually help Koizumi overcome her emotional insecurities. Despite maintaining his overtly sexual demeanor, he demonstrates genuine care, such as valuing her friends' support and reacting emotionally when she acknowledges their bond. His development culminates in Koizumi crediting him as a key reason she stays in Japan.

He participates in school basketball, joining initially to meet girls but displaying unexpected skill. His friends Urawa and Kuwabara often encourage his pursuit of Koizumi, though their plans frequently backfire. His home life includes a mother who observes his personality shift. Physical comedy punctuates his actions, with Koizumi frequently punching or kicking him into the air as punishment for transgressions.