Many clients in response to a charge bring in stacks of letters testifying to their good character. In addition they point to a lack of previous arrests in defense of themselves. Character evidence has a place in criminal procedure but it never is a defense to the crime charged. Can you image Lee Harvey Oswald claiming he was never arrested until he shot JFK? Yeah but so what? And that's the problem with most so-called character evidence. It doesn't prove you didn't do the crime.
What Can It Be Used For?
In CA character evidence can be used to show some fact, such as motive, opportunity, intent, preparation, plan but not to show a predisposition to commit the crime. Furthermore evidence of good character can be used at trial only by the defendant to prove conduct in conformity with a character trait or by the prosecution to rebut such evidence used by the defense.
What Good Is It Then?
It's important for certain crimes, such as sex crimes or domestic violence crimes but beyond that it's primary use is at sentencing to try to convince the judge the convicted defendant deserves a break. It usually has little effect on the sentence imposed.