Natsuko is Mahito's stepmother, married to his father Shoichi following the death of Mahito's biological mother in a Tokyo hospital fire. She lives with them on a rural estate housing Shoichi's factory and is attended by elderly caretakers. Her pregnancy signifies Shoichi's drive to form a new family unit after their move.

Natsuko shares a reserved relationship with Mahito, who struggles with unresolved grief and resentment towards his father's swift remarriage. Her disappearance into a mysterious tower on the estate—built long ago by a vanished relative—acts as the catalyst for Mahito's journey into a fantastical realm. Driven by duty despite their emotional distance, Mahito pursues her through this gateway.

Her role centers on her symbolic connection to Mahito's trauma and her function as the impetus propelling his exploration of the supernatural world.