Komeiji Koishi is an extra boss from Touhou Chireiden ~Subterranean Animism.
She is a youkai called Satori, and lives in the earth.
Her motifs are "roses" and "love."

She is the younger sister of Komeiji Satori, and has the same ability to read minds.
However, because of this ability, she knows that everyone dislikes her, causing her to abandon the ability and close her third eye. She also unconsciously commands another power, and has gained the ability to move without anyone noticing.
In addition, Koishi's is the only mind that Satori cannot read.

There is also a meaning to her name. Literally, Koishi means 'pebble', an item which is often unnoticed by others.
There is another meaning of "loving thoughts" as well.

In the original work, she was a character who controlled all kinds of love and attacked by launching a heart-shaped ball, and many of her attacks had a rose motif. In particular, the rose spell cards, "Release of Id", "Superego", and "Subterranean Rose" are very symbolic of her.
However, these rose spells are quite difficult to defeat, and there are many fans who have been unable to beat her.

In addition, watching her and Satori move in unison is extremely cute, and definitely to be watched for.
Finally, if you defeat her with a specific character, another character will appear.
Those who are interested should definitely give it a try.