Jun'ichi Nagase, a high school sophomore, carries the lingering reputation of his junior high delinquent persona, "Geno Killer." His life began to change when his younger sister Minato moved in with him, prompting a gradual mellowing. Their parents, working as frequently absent spies, leave Jun'ichi and Minato to live independently.

A dramatic shift occurs after Jun'ichi rescues transfer student Yuuhi Katagiri from harassers. Misreading her demeanor, he kisses her publicly at school, sparking her intense fury. This conflict escalates when both families reveal an arranged engagement between them, forcing Yuuhi to temporarily move into Jun'ichi's home. They must date for one month to reconcile or annul the engagement.

Jun'ichi exhibits a snarky attitude and endures frequent teasing from peers and teachers. His relationships drive significant development: he grows protective of Yuuhi despite their clashes, while his interactions with Minato blur familial boundaries. He experiences romantic fantasies about Minato, initially denying these feelings. His romantic conclusion diverges across adaptations: the anime ends with him confessing his inability to live without Minato, choosing her over Yuuhi, while the manga concludes with him reciprocating Yuuhi's feelings and entering a relationship with her.

The precise nature of his sibling bond with Minato varies by media. The original PC game identifies them as blood relatives, whereas the PS2/PSP ports and anime imply Minato may be adopted or unrelated, without definitive confirmation. In the OVA, he joins classmates on a vacation to a private island, participating in comedic scenarios without major character evolution.

His background includes covert tendencies, such as hiding a porn stash, and comedic reactions to others' antics, like the student council president's schemes. Despite his delinquent past, he demonstrates care for those close to him, mediating conflicts within his social circle.