The credibility of any witness is always relevant. Demand the criminal history with a discovery demand, and motion to suppress testimony if not produced.
We do not have a client/attorney relationship until you make an appointment, we discuss your case face to face, I accept a retainer, and we explictly agree to enter into representation.
Kentucky state law on discovery and evidence may control the answer to your question, but it is certanly possible that the defense could obtain access to, and use, at least some of this information. The defendant in this case needs an attorney, without question.
If the prosecution witness has received any benefit from the prosecutor ( a lesser sentence, agreement not to be charged, not to be revoked, etc.), in exchange for their testimony, those facts are clearly admissable in trial and exposure on cross-examination by the defense attorney. U.S. Supreme Court case law and KY case law allow exploring these items on cross-exam to show any motive, bias or consideration extended based on a favorable agreement with the government.
I am trying to give you a general answer to your question. We do not have an attorney-client relationship by this response on the avvo website. I have not been retained to represent you. I am licensed to practice law in Kentucky and in federal court in this state and the Southern District of Indiana. You need to seek legal advice from an attorney licensed to practice in your area..