Miko is a nineteen-year-old girl working part time at a local shrine as a shrine maiden. She claims that her name, "Miko," corrisponding with her job as a shrine maiden ("miko" in Japanese) is a coincidence. At the beginning of the story, she is involved in the hunt for the Platonic Heart, fueled by the wish to have true friends. She later drops out of the contest after meeting Aya and Natsume because of the fact that they become her first friends.
She later goes missing, thus causing Aya to join the contest for the Platonic Heart in hopes that if she wins, Miko will be found.
In the final episode, Miko appears to Aya after she defeats her final opponent. She takes Aya into a building, shows her all of the opponents that she had defeated, and then reveals that she is one of the heads of the Platonic Heart along with Natsume and her brother Haruki, whom Miko reveals to be her lover. Miko tells Aya that she only pretended to be her friend in order to get her to join the Platonic Heart contest, which was only set up in order for her, Natsume, and Haruki to get their revenge on Aya. The cause of Miko's strong hate for Aya was fueled by the fact that Aya's father is her father as well, making them half-sisters. Their father apparently beat Miko's mother, who was a mistress, and was the one that killed her. Miko also explains that her mother and her sister, which is Natsume and Haruki's mother, were competitors in the Platonic Heart contest years ago, which turned out to have been established by Aya's father. Since Miko's mother lost, her punishment was to become Aya's father's mistress, while Aya's mother, who was the winner, became his wife.
It is later revealed by Aya's mother that the whole reasoning behind everything was because Miko's mother's father, which would be Miko, Natsume, and Haruki's grandfather, killed both Aya's mother and father's parents. Out of a lust for revenge, Aya's father established the Platonic Heart contest to get Miko's mother and Natsume and Haruki's mother to participate in it, that way, he could get revenge on their father by doing harm to them.
