Explain xkcd: It's 'cause you're dumb.
Hairy is a stick figure character in xkcd. The name is unofficial, used by explain xkcders to describe male characters with hair and no other distinguishing features.
Hairy is not a single character but a categorization of various one-time characters. One Hairy, a learning pickup artist, has appeared in two comics (1027: Pickup Artist and 1178: Pickup Artists).
When Hairy comes up against Cueball or Black Hat, he is most likely to come off worse (as in 1210: I'm So Random, or the pickup artist Hairy). Hairy can also appear just to add some variation in a group (1261: Shake That), or as a kid (973: MTV Generation, 1176: Those Not Present).
There is a very early comic (16: Monty Python -- Enough) where it is a hairy guy who delivers the joke. But there is no reason to believe this is the same Hairy as Randall invented later.