Taking actions unconsciously
As her actions are controlled entirely by her subconscious, Koishi's presence cannot be noticed by other beings unless she allows it, even while standing in plain view because she's closed her mind, or her Third Eye. Likewise, her older sister's ability to read minds has no effect on her, as she does not technically have a mind to read; that's why she said, "my sister can't win against me." Even someone who manages to communicate with Koishi will forget about her after she leaves, unless they already knew of her as Satori's sister.
Some children are immune to this effect due to their still-developing minds, leading Koishi to become their "imaginary friend", though they will forget about her when they grow up. Other than making others unable to perceive her existence, it also makes the act of doing so difficult to perceive. Practically, without being noticed by tengu or the other youkai, she climbed Youkai Mountain. Even the optical camouflage was spoiled.
The youkai satori, on whom she and her older sister are based, by the actions of the humans' unconscious, they saw painful experiences. Due to this it is also not known if this ability was spoken of cynically in the past. However, since she closed her third eye which can read minds, she became unable to read others' minds. As she said herself, "reading people's minds is depressing, there's nothing good about it."
Manipulation of the Unconscious
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.