Miko Kazuki serves as a shrine maiden whose name mirrors her role. Initially entering the Platonic Heart tournament seeking friendship, she withdraws after bonding with protagonist Aya Iseshima and Natsume Honma, declaring her wish for companionship fulfilled. Her sudden disappearance drives Aya to join the tournament in search of her.

Miko resurfaces as a primary antagonist in the final episode, revealing her participation was a ruse to manipulate Aya into the competition. She co-masterminds the tournament with Natsume and Haruki Honma to exact revenge against Aya’s family, particularly her mother Suzuko Iseshima, for past atrocities tied to the event.

Her motivations stem from familial betrayal: Miko is Aya’s paternal half-sister, both fathered by Shigeyuki Iseshima. Miko’s mother, Yumi Kazuki, was Shigeyuki’s mistress, while Aya’s mother Suzuko was his wife. This arrangement resulted from an earlier tournament where Yumi and her sister Kumi Honma (mother of Natsume and Haruki) lost to Suzuko and were forced into sexual slavery. Shigeyuki murdered Yumi on Miko’s fourth birthday, igniting Miko’s lifelong vengeance.

Collaborating closely with the Honma family to revive the tournament, Miko oversees the enslavement of defeated contestants through mind-altering techniques, reducing them to compliant sex slaves—a fate she hypocritically blames on Aya despite orchestrating it. Her romantic bond with Haruki Honma solidifies her alliance with the conspirators.

The revenge cycle spans generations: Miko’s maternal grandfather killed Suzuko and Shigeyuki’s parents, prompting them to create the tournament against Yumi and Kumi. After Kumi escaped and killed Shigeyuki, she manipulated her children and niece Miko to target Suzuko and Aya.

In the climax, Suzuko confronts Miko, Haruki, and Natsume in their hideout. After recounting the family history, Suzuko kills all three in self-defense as they attack her. Suzuko subsequently perishes in the burning building, concluding Miko’s story.