A more thorough explanation:

Character evidence refers to evidence that describes an individual's personality traits, moral standing, or tendencies. In criminal cases, character evidence is generally not admissible unless the defendant brings it up first. This is because character evidence can be very persuasive and may unfairly influence the jury's decision.

For example, if someone is accused of stealing, evidence that they have a history of stealing may not be admissible unless the defendant brings up their own character as a defense. This is because the evidence may unfairly influence the jury's decision and cause them to assume that the defendant is guilty based on their past behavior.

Similarly, evidence that the victim has a history of violence may be admissible if the defendant claims that they acted in self-defense. This is because the evidence may help to show that the victim was the aggressor and that the defendant was acting in self-defense.

Overall, character evidence is generally not admissible in criminal cases because it can be very persuasive and may unfairly influence the jury's decision. However, there are some exceptions to this rule, such as when the defendant brings up their own character as a defense or when the evidence is used to show that the victim was the aggressor.