Rule 790 Evidence of the good character of a witness is inadmissible to support his credibility unless evidence of his bad character has been admitted for the …

If someone testifies in court, the other side cannot bring in witnesses or evidence just to say that person is honest or trustworthy. Good character evidence about a witness is not allowed simply to make the jury believe them more.

There is one exception. If someone has already attacked a witness by bringing up their bad character, then the other side can respond by bringing in evidence of that witness's good character. Basically, you can only defend a witness's reputation after it has been attacked.