Mr. Morrison is basically the Logan family bodyguard.  He is brought in by Cassie’s father to protect the family during the racial conflict.

Mr. Morrison is a strong, honest man.  He explains to Mrs. Logan that he got fired from his previous job for fighting.  He got fired even though the other man started it because the other man was white (p. 37).  She does not hold it against him.

Mr. Morrison was a quiet man, almost shy, and although Mama had offered him lodging in our house, he preferred the old shack. (p. 77)

Mr. Morrison helps around the house, and serves as protection when the family needs it.  For example, he goes along with Cassie’s father to investigate what happened with the Averys.  He also keeps watch after her father is attacked.  In the end, they are safer for his being there, but he does not act as instigator.  He is their protector, but he stays out of the fight unless he has no choice.