Natsumi Hayama, daughter of Fuyuki Hayama and the late Koharu Hayama, serves as the older sister to Akito Hayama. Her mother's death during Akito's birth ignites Natsumi's lifelong resentment toward her brother. She blames Akito for their mother's demise, expressing this through persistent mistreatment—verbally abusing him as a "demon" who should never have been born, and repeatedly expelling him from their home physically.

Natsumi deepens the family's fracture by deceiving Akito, falsely asserting their father shares her hatred. This lie isolates Akito from their father, cementing a dysfunctional household where interaction and shared meals vanish. Her hostility, extending beyond Akito, manifests an unforgiving and vindictive nature.

A transformative moment strikes when Natsumi watches a television movie by Sana Kurata, a child actress she idolizes. The film mirrors her own family's conflict, depicting a sister who condemns her brother for their mother's death. This parallel jolts Natsumi into recognizing her cruelty toward Akito. She reconciles with him, halts her abuse, and actively rebuilds family bonds—cooking meals and joining shared activities like television viewing.

Post-reconciliation, Natsumi evolves into a supportive, occasionally overprotective sister. She and Akito regularly watch Sana Kurata's show *Child's Toy* together, reflecting her enduring fandom. She diligently tracks Sana's updates through magazines and newspapers. Her arc culminates in full family reintegration, where she displays consistent warmth and responsibility toward Akito and their father.