Uncle Hammer's guiding principles have been forged by his experiences dealing with racism. He is Mary Logan's brother who lives in Chicago. Unlike his southern relatives, Hammer appears as a more "well-to-do" character. He's polished, urbane. He drives a beautiful Packard (almost identical to Logan nemisis, Harlan Granger), wears nice clothes, and carries himself with a certain swagger, born of pride and prejudice. When Hammer hears that his niece (and protagonist/narrator, Cassie Logan) has been mistreated by Mr. Simms, he isn't willing to "turn the other cheek."
Hammer fought in World War 1. He's embittered about his experiences, calling it a "white man's war." He's lost much and isn't willing put up with the types of injustices that his Logan relatives have come to grudgingly accept.
His principles include standing up for yourself. Unfortunately, his experiences have somewhat clouded his judgement. He isn't willing to compromise. He's "hot-headed" in his reaction to any sort of injustice perpetrated against himself and his family.
Basically, Hammer wants to protect the Logans and fight racism whenever it is possible.