Bakemonogatari - Anime Complete Guidebook

One day, high-school student Koyomi Araragi catches a girl named Hitagi Senjougahara when she trips. But—much to his surprise—she doesn’t weigh anything. At all. She says an encounter with a so-called “crab” took away all her weight … 

Monsters have been here since the beginning.
Always.
Everywhere. 

(Source: Kodansha USA)