Kosuke Tamura, brother of Tamura Productions chairman Yuusuke Tamura and uncle to protagonist Eriko Tamura, seizes control of the company after a car accident kills his brother and leaves Eriko's mother Minako comatose. He becomes Eriko's primary antagonist, systematically working to disinherit her and seize her remaining assets while sabotaging her idol career through financial manipulation and incentivizing others to undermine her public image and professional opportunities.

Kosuke manipulates Eriko's rival, Rei Asagiri, by falsely claiming Yuusuke is Rei's father who abandoned her. This scheme preys on Rei's insecurities from her mother Ryoko Asagiri's earlier abandonment. The narrative later exposes Kosuke as Rei's actual biological father—making Rei and Eriko cousins—though he conceals this truth, and Ryoko refuses to identify Rei's father when questioned.

His tactics grow increasingly severe. He orchestrates a physical assault on Eriko in Guam, hiring locals who destroy a keepsake from her father before intervention halts the attack. Kosuke also engineers a tabloid scandal falsely branding Eriko promiscuous, triggering public harassment and deepening her emotional turmoil.

Despite prolonged antagonism, Kosuke experiences a redemption arc. After Minako awakens, he aids her in reclaiming Tamura Productions from his own influence. Post-reform, he adopts a changed demeanor, humorously assisting Eriko by carrying her dogs Fortissimo and Pianissimo, ultimately resolving his conflict with the protagonists.