Patchouli Knowledge is a friend of Remilia's and spends most of her time in the Scarlet Devil Mansion library. In fact, it wouldn't be a stretch to say she practically lives in the library. She'd found herself engrossed in yet another book when Koakuma came along with a rare volume she'd found while tidying.

Koakuma is Patchouli's familiar and is an extremely hardworking devil. It's said Koakuma (meaning small devil) isn't her official name but was given to her based on her petite appearance. As she was tidying the library yet again, she stumbled upon a rare book. She ran over to Patchouli to report her discovery, but all of sudden, the book began to glow.

In general, libraries are public facilities that collect, organize, and store books. In recent years, they've also started allowing visitors to use computers and borrow comic books. However, since the library Patchouli manages is in the basement of the Scarlet Devil Mansion, only a select few people ever visit. That includes Remilia and Sakuya, as well as Marisa, who seems to drop by almost every day.

The book Koakuma found contained spirits that had been sealed away. When she moved the book, the seal broke, and the spirits trapped inside burst out! Patchouli and Koakuma fought bravely and managed to re-seal the spirits... but the battle had taken its toll on them.

Sakuya and Meiling came running when they heard the sound of bullets. When they reached the library, they spotted Patchouli and Koakuma, who now had animal ears and tails. According to Patchouli, the spirits had cast a curse on them in the moments before they were re-sealed. She was certain they'd both be back to normal in a few days... but the library had far more visitors than usual in that time.