Koishi is generally playful and free-spirited, not unlike a child. While her ability to manipulate the unconscious makes her presence unsettling to many people, she seems unconcerned with appearing scary, and does whatever feels right in the moment. She does not seem to go out of her way to harm others out of malice or spite, although she has occasionally caused issues for other characters due to a lack of self-awareness, such as in her accidentally stealing Hata no Kokoro's mask in Hopeless Masquerade, or Sumireko Usami's smartphone in Urban Legend in Limbo.

In Urban Legend in Limbo, she takes on the urban legend of Mary-san. Rather than wanting to be truly menacing, however, Koishi appeared to "join in on the fun" with the other characters also performing urban legends, eventually having to receive advice from Marisa Kirisame on how to make her urban legend scarier. She seemed genuinely happy that she was able to use her urban legend effectively, even if it was only on a single person.

Having purged herself of "worldly attachments," she's viewed as having achieved something akin to enlightenment by the Buddhists of the Myouren Temple, although they question whether or not the price she paid was in her best interests.

As her actions are controlled entirely by her unconscious, 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, which is 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. However, as Satori's presence has been made generally known among the youkai of Gensokyo, Koishi has begun to be recognized by other residents as of late.

The youkai satori, on whom she and her older sister are based, by the actions of the humans' unconscious, they saw painful experiences. Since she closed her third eye which can read minds, she became unable to read others' minds.

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.

Koishi currently resides at the Palace of the Earth Spirits, but she is also a laywoman in regards to the Myouren Temple as a recent convert to Buddhism, at Byakuren Hijiri's request.

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.

In Subterranean Animism, Koishi has wavy, green or greyish, short hair and dark green eyes, wears a black hat with a yellow ribbon around it. A closed, blue-purplish "third eye" is positioned over her heart, with cords wrapped around her torso and attached to each of her ankles. She wears a yellow shirt with diamond-like blue buttons and dark green collar. The skirt is also green with a faint flowery pattern, but is lighter. Her socks are black. As of Hopeless Masquerade, she has bright green hair, and dark green eyes with white pupils. Her "third eye" is also more-towards a purple colour, and is more-clearly shown to wrap around her waist.

Satori's and Koishi's designs may have been inspired by the 1939 painting by Frida Kahlo titled "The Two Fridas". The subject is essentially two identical women (sisters/clones/twins...) with visible hearts and external arteries that seem to connect and wrap around their shoulders and arms. One of them has a torn heart and an artery cut by a pair of scissors in hand (resembling Koishi, who has closed her third eye).

Interestingly, if you reverse Koishi's color palette, the resulting palette is similar to Satori's. Similarly, if you reverse Satori's, the resulting palette is similar to Koishi's.

Koishi's most important relationship is her older sister, Satori Komeiji. Despite Koishi closing her mind and seemingly whimsical wandering, as well as the fact that even Satori has trouble "seeing" her, Satori does worry about her.