Bakemonogatari

Koyomi Araragi, a high-school student, one day comes to the rescue of a girl named Hitagi Senjougahara who tripped. However, he is astonished to find out that she has no weight at all. Hitagi explains that her weight was taken away during an encounter with a creature known as the "crab." Since the dawn of time, monsters have existed, and they persist everywhere.