Bakemonogatari (Manga), volume 1 - NISIOISIN

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.