Papa brings Mr. Morrison to live with the Logans so the family can have some kind of protection while he is away working. He's described as: a "human tree [...] towering high above Papa's six feet two inches. The long trunk of his massive body bulged with muscles, and his skin, of the deepest ebony, was partially scarred upon his face and neck, as if by fire" (2.14).
We find out later that Mr. Morrison is scarred because he was almost burned to death as a child. His mother (killed along with her father by "night men") threw him out of the burning house before the flames could consume him. So, it makes sense that Mr. Morrison comes to regard the Logans as his surrogate family by the end of the book.
Morrison may be quiet and shy, but he's also a man of action. Kind of like a cat (one of the dangerous ones), and Taylor describes him like this more than once:
The moon slid from its dark covers, cloaking the earth in a shadowy white light, and I could see Mr. Morrison clearly, moving silently, like a jungle cat, from the side of the house to the road, a shotgun in his hand. (3.212)
He protects Papa with brutal force when the Wallaces harass them on the way back from Vicksburg. Check out what Stacey says about Mr. Morrison's super-heroic actions:
I did see Mr. Morrison pick up one of them men like he wasn't nothing but a sack of chicken feathers and fling him down on the ground so hard it must've broke his back. Ain't never seen nothin' like it before in my whole life. (9.177)
From the first time Taylor introduces him, Mr. Morrison's voice sounds like thunder: his voice is a "deep, quiet voice like the roll of thunder" (2.34). This foreshadows not only the violence that he (Mr. Morrison) commits against the Wallaces in self-defense, but also the tragic events at the end of the book—which take place during a storm.
Maybe we can think of Mr. Morrison as the system destroying itself. Racism destroyed his family—and he's not afraid to fight back.