Bakemonogatari Vol 1: Hitagi Crab(化物語)

On the day after the Golden Week holiday, third year high school student Koyomi Araragi finds himself saving a classmate, Hitagi Senjyogahara, by catching her when she slips on the stairs and comes tumbling down from above. He discovers that she's impossibly light, as if she has no physical weight whatsoever.