ANSWER:
You have no right in exposing her sins if she had repented from them! If she used to hangout with boys for example but she repented and started wearing hijab and she is a good woman, you must not say a word about her past.
But if you don’t know whether she stopped or not, you may just say that you do not recommend her for marriage without any elaboration.
As for her character, if you know that she has rage problems, you should disclose this or if she has mental problems, but without details, just general.