Patchouli Knowledge is the brains of the Scarlet Devil Mansion. It would not be an exaggeration to say that she is the one who solves all the troubles that occur within the Scarlet Devil Mansion.

A magician is someone who has been bewitched by magic and become a youkai with a body that runs on magical energy. Alice quit being human and became a youkai through the culmination of her studies. Marisa is a human who has learned how to use magic through her studies. Patchouli is about the only one who's a genuine magician from birth.

Magic books are not just ordinary books, they are items infused with magic. The great library houses countless magic books―it's said that there are even books written by Patchouli herself. Koakuma takes care of and arranges all the books.

You surely already know this, but the great library is in the basement of the Scarlet Devil Mansion. It is extremely dark, quiet, dusty and smells of mold. Not a good environment for Patchouli, who has had a weak constitution since birth. However, the books may not be the only reason why she continues to reside there. It may be because Remilia, who is weak to sunlight, can visit her there in the daytime.

If you ever happen to visit the great library with Marisa, you're sure to see the results of Patchouli's magic studies... in the form of dazzlingly beautiful bullets that chase out intruders. As you watch Marisa try to escape the bullet hell from the sidelines, Patchouli is sure to give you a friendly warning. "Don't be like this little black rat. Just come and visit me normally if you want to read any books. Though I can't promise I'll let you read them."