Crabbe is one of Draco's two stupid henchmen. He follows Draco everywhere looking vaguely threatening. He is such a fool that, when Harry and Ron leave two chocolate cakes on the banister near the door to the Dining Hall, Crabbe gobbles the cake up without thinking. This turns out to be a bad idea from Crabbe's point of view, since the cake contains a knockout potion to allow Ron to get some of Crabbe's hair for his Polyjuice Potion. Goyle, of course, makes the same mistake; Harry turns into Goyle. Not that there's a big difference between Crabbe and Goyle; the two seem basically indistinguishable from one another.
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