Bakemonogatari, Volume 1 (Bakemonogatari: Monster Tale Manga, #1)

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.