Grey Beaver isn't strictly an antagonist, since he doesn't hate White Fang or try to kill him. And he's got a little bit of mentor in him too, since he domesticates White Fang and teaches him the virtues of tasty treats. But he certainly isn't White Fang's buddy. He hurts him when he's been bad and uses him more as a living tool than something worthy of respect. That most likely makes him a foil—someone who teaches White Fang important things through a pain-is-the-cleanser training regime that isn't entirely welcome.