A character witness is a witness in a hearing or trial who vouches for the defendant's character. They usually don't have anything substantial to say about the facts of the case - merely that their friend couldn't possibly have done it, because they are an upstanding citizen too busy saving kittens from trees and feeding the homeless to possibly have embezzled thousands of dollars or garroted their boss.
I served as a character witness once for a friend who was on trial for stealing a car. I told them about the time he had helped my mother pay the rent, and how I didn't see him as the type of person who would ever do something risky like stealing a car. I conveniently left out the time we had pulled a minor insurance fraud scam by both vouching that his foot "injury" had been caused on the job, and not horsing around (the truth). He was acquitted, of course; I don't know if he actually stole the car.
I guess in theory you could be called up as a character witness against someone ("Well I don't know if he did it, but I wouldn't be surprised if he garroted his boss"). It seems that a character witness would be the weakest kind, after eyewitnesses and expert witnesses. Any criminal lawyers or trial scholars beg to differ, let me know.
eliserh sez: Actually, you can only be called up as a character witness against someone if they have already called character witnesses in their favour.
I don't know if I would trust her - she spells favor with a u. Still, they know better than I do, I'm sure.