We are first introduced to Uncle Hammer in Chapter Six of this excellent novel, when the children come back to find a car that looks remarkably like Mr. Granger's car parked in their barn. Note how Uncle Hammer is presented as we are introduced to him:
Instead of Mr. Ganger, a tall, handsome man, nattily dressed in a grey pin-striped suit and vest, stood by the fire with his arm around Big Ma... Like Papa, he had dark, red-brown skin, a square-jawed face, and high cheekbones; yet there was a great difference between them somehow. His eyes, which showed a great warmth as he hugged and kissed us now, often had a cold, distant glaze, and there was an aloofness in him which the boys and I could never quite bridge.
As his name implies, there is a violence in Uncle Hammer's character against the racism that he and his family has to endure. This is of course why Mama does what she can to prevent Uncle Hammer from finding out about the way Cassie was slighted by Lillian Jean and her father. Notice how when he hears about this, his eyes "narrowed to thin, angry slits." He is a character that therefore captures the anger and rage of the way that blacks are treated by whites in this novel.