Mitsuko Nanase works as an office employee under Yōji Yagami, Yuuji's father, at his company. She harbors intense romantic feelings for her boss Yōji and persistently pursues an affair with him, disregarding his marriage. Her jealousy erupts after encountering Yōji's wife Nomi, fixating on Nomi's youthful looks; Mitsuko confronts Yuuji to demand whether she or Nomi is more beautiful. Visually, she often displays a sharp, intimidating expression that Yōji mistakes for anger.

Her involvement escalates when she accidentally strikes Yuuji with her car. While transporting him to the hospital, she discovers his identity as Yōji's son. This revelation spurs her to ally with Yuuji, suggesting they sabotage Yōji and Nomi's marriage together—aiming both to remove obstacles to her own romantic goals and to aid Yuuji's unresolved maternal sentiments. She engineers private meetings with Yōji, crafting scenarios to provoke infidelity.

Yōji, however, remains oblivious or indifferent to her advances, showing no reciprocal interest. One arranged encounter collapses when Nomi unexpectedly intervenes, prompting Yuuji's regret over their scheme upon witnessing his mother's distress. Later, Mitsuko redirects her romantic focus toward Yuuji, proposing a visit to a love hotel, though this pivot receives minimal exploration. She functions primarily as a disruptive element within the family dynamic, with no detailed background or resolution provided.