A character witness is an individual who testifies in court to verify the truthfulness of claims made by another witness or, more commonly, a defendant in the legal case. The role of the character witness is to certify that the testimony given by the defendant has been honest and in accordance with their community status, and it's often utilized in cases where the honesty of the testimony can be questioned - for example, when the case involves a high-ranking criminal or another individual prone to giving false testimony.
A common example of character witnesses are celebrities and reputed community members, who may be summoned by a defendant to certify that the defendant in question is honest and truthful, and to additionally reinforce their social status and the nature of their actions in the public eye. In some notorious cases, inmates on death row have summoned other death row inmates as their character witnesses, and there have been numerous cases where this has proven beneficial to the defendant's case and their representation in court.
In most cases, the character witness will perform their testimony by outlining several positive traits of the defendant in question, adding how they're absolutely sure of their honesty and rightfulness, and additionally providing any testimony which may aid the defendant in their case (for example, personally testifying that a certain statement is true due to them having observed the action in question). The admittance of character witnesses varies, and some courts disallow them completely depending on the case.