The Role of Champions in a Trial by Combat

Some people used champions to fight in their place. What were the rules around that? Would the champion be somebody related to you, or might it be somebody that you hired? For example? I think most often somebody that one hires. And sometimes it will be somebody who's a particular supporter for one reason or another,. Like a a nephew or a brother or something. But there are proe onal champions as well, who will fight on behalf of people. Seems like a fairly lousy job to do, really, for all sorts of reasons.