She is Satori Komeiji's younger sister. She wanted to avoid the hate and dislike which Satori experienced due to her mindreading abilities, so she sealed away her own Third Eye. As a result, she lost her ability to read others' hearts, but in return she gained the ability to take action unconsciously. The denizens of the underground no longer hated her, but at the same time, they never feared her and animals no longer loved her.
The ability to read people's hearts also makes one's own heart stronger.
Closing off that ability because it's hated is just running away, and is no different from closing off one's own heart. It's effectively pushing away others' feelings and shutting down. She became a youkai who wandered to and from aimlessly. Personally, she thought it was fun, so it wasn't really a problem. People around her felt a little sorry for her. Even her older sister, Satori, is unable to read Koishi's sealed-away heart, and doesn't really know where she goes or what she does there.Just like a pet cat, she leaves and returns and leaves again according to her own whims. Satori took pity on Koishi, ordering her pets to play with her and giving her some pets to keep as her own. She thought that taking care of the pets might give her a purpose, and it might enable her to begin opening her heart to others again. Indeed, it seemed that Koishi began to change, ever so slightly, after she began to raise her pets. She heard that a human had come from above ground and had fought with her sister, Okuu, and the other pets, and was amazed at the incredible power-up Okuu had gotten, so she decided to go see the above ground world herself. Because she could act unconsciously, she remained unnoticed. She could go above ground, sneak past sleeping shrine maidens, or go into the mountain the tengu guarded, and nobody would take notice of her. In a way, it was convenient, but she was also lonely. However, she had no heart capable of feeling this loneliness. At her destination, the shrine atop the mountain, she happened to meet a human. Her meeting with this stranger changed her mental state ever so slightly.
She thought, how amazing it is to meet such an interesting person, and wanted to know more about her. For the first time, she regretted having closed her third eye.
Vova
