Don't hate Mr. Jamison just because he's an attorney. There's a lot more to him than that.
In fact, Mr. Jamison is one of the few sympathetic white characters in the book. He rates so highly in Cassie's book that she calls him "our Mr. Jamison" (4.226). Why does she like him so much? Here's one reason: "He was the only white man I had ever heard address Mama and Big Ma as 'Missus," and I liked him for it" (5.25).
So, Mr. Jamison is one of the good guys. He genuinely wants to help the poor black farmers, and even offers to back their credit when they're planning the boycott of the Wallace store (7.149).
But he offers more than legal advice. He's also fairly heroic. When the lynch mob is about to hang T.J., it's Mr. Jamison to the rescue. He rolls up onto the scene and before his car even stops he "leaped out" and "stared at each of the men as if preparing to charge them in the courtroom" (11.77).
Right about now you're probably thinking: "Wow...he sounds like Atticus Finch." Indeed, he does. There are several parallels worth considering: both are lawyers, and both are staunch supporters of justice. They're also both sympathetic to the plight of African-Americans and the injustice inflicted upon them. Can you think of any other parallels?