Seiya Kubo occupies the role of Reika Kawatani’s boyfriend, presenting himself as a freeloader who clings to aspirations of becoming a professional musician while steadfastly avoiding stable employment or dedicated musical practice. Their relationship ignited when Reika became captivated by his guitar performance, only to erode under his manipulative tendencies and neglectful conduct.

Plagued by a compulsive pachinko addiction, he habitually pilfers Reika’s college savings, invading her apartment to seize her bank passbook and manipulating her into surrendering funds through fabricated promises of musical collaboration or job-seeking efforts. His behavior reveals a controlling streak, brushing aside her emotions while relentlessly prioritizing his own indulgences.

The dynamic collapses when Seiya escalates his exploitation by pressuring Reika into a questionable part-time opportunity to fund his habits—a confrontation halted by Mire Yoshitsuki’s forceful intervention. Empowered by this clash, Reika reclaims her apartment keys and decisively severs ties. Though initially charming enough to keep Reika indecisive, Seiya’s unyielding refusal to reform cements his role as a destructive presence until their relationship conclusively ends.