Mr. Grimwig is a typical Dickens character: he's eccentric, and his eccentricity takes the form of a frequently repeated verbal or physical tick. His favorite expression is, "I’ll eat my head!"—which we're totally stealing—and he repeats that phrase so often, and so oddly, that that’s pretty much all there is to his character.
He’s reducible to his own eccentric expression. Sure, he has other characteristics—he’s stubborn, contrary, abrupt, hard on the outside, but marshmallow-y soft on the inside, and very fond of Rose (who isn’t?). But he’s memorable primarily as the "I’ll eat my head!" guy, and that’s one of the striking things about how Dickens writes minor characters—they’ll often be reducible to some odd or eccentric expression or gesture.