Koishi Komeiji is a satori and the younger sister of Satori Komeiji. She stars first as the extra stage boss for Subterranean Animism, and then later as playable characters for Hopeless Masquerade, Urban Legend in Limbo and Antinomy of Common Flowers.

Look I like Koishi Komeiji, you know I had to make a shrine out of it. It beat out the 2000 United States presidential election shrine I thought of doing as well.

Koishi Komeiji is a playful and free-spirited character, often compared to a carefree child. While her ability to manipulate the unconscious can make her unsettling to others, she doesn't intentionally aim to be scary and simply acts on her impulses. She rarely causes harm out of malice, though her lack of self-awareness sometimes leads to trouble, such as stealing Hata no Kokoro's mask in Hopeless Masquerade or Sumireko Usami's cellphone in Urban Legend in Limbo. Koishi's closed Third Eye makes her actions controlled by her unconscious, rendering her unnoticed by others unless she wills it—even while standing in plain view. Satori's mind-reading ability has no effect on her, as Koishi technically lacks a readable mind, explaining her claim that "my sister can't win against me." Most who meet her forget about her once she leaves, except for those aware of her as Satori's sister.

Children, due to their developing minds, sometimes perceive her as an imaginary friend, though they forget her upon growing up. Her unnoticeable presence allows feats like climbing up Youkai Mountain unseen. However, as Satori has become more known in Gensokyo, Koishi has also begun gaining recognition. She has abandoned her mind-reading ability, finding it depressing, remarking that "reading people's minds is depressing, there's nothing good behind it." Koishi's presence cannot be "felt" by anyone unless she has entered their direct field of vision and is forgotten by most people as soon as she leaves it. She can manipulate the unconsciousness of every living thing around her, allowing her to make them see everything she desires and make them confront their own weaknesses hidden inside their hearts, as seen in lot of her Spell Cards. To escape the fear and hatred which other beings feel towards the satori species, she attempted to destroy her mind-reading ability by closing her Third Eye. However, this had the side effect of sealing away her own conscious mind, causing her to lose all thoughts and motives; she could no longer be hated, but neither could she be loved, or even remembered by people who saw her.