The Dragon hates noises inside the library. If the dragon catches any Human being too loud, The Dragon will attack the offending party with Eight of it's heads, and anyone who gets caught will be tossed out of the library and into a pond facing the entrance.
Pac Dragon is a massive, reptilian creature with dark purple skin. Its body is rather bulky and it crawls using two four-fingered hands with white claws. It has a long, thin tail and each time one of its heads are cut off, three more grow to replace it. Its number of heads can apparently grow is the maximum of eight similar to the Yamata no Orochi.
When Team Okaina, Team Takashi and Team Kyoji accidentally make a creak in the library's floorboards, grabbing the dragon's attention, they all start scattering and purposely make sounds from different parts of the library, therefore confusing and distracting the librarian while Takashi gets the Shinto Books unnoticed.
The Pac Dragon is based on the Greek Mythogical monster: Hydra.