A satori and a natural telepath, Koishi has sealed off her mind-reading ability (represented by the fact her third eye is closed) so she would not experience the fear and loathing her sister suffered. This indirectly gave her a new and even more terrifying power, the ability to read and manipulate the subconscious mind. Her own conscious mind was sealed away as well, and she no longer has a real personality or desires. Koishi now wanders Gensoukyou, going wherever her subconscious wills her. She is the younger sister of Komeiji Satori.
Koishi is portrayed with either silver or green wavy hair and green eyes. She wears a yellow collared shirt with long and wide sleeves with frills and a green skirt with floral print. On her head, she wears a black hat with a yellow ribbon around it. She is easily identified by her blue third eye, an external eyeball that is wired to her clothes by veins/cords (often forming a heart of string).