What does “real party in interest” mean when being sued for credit card debt?

Real party in interest simply means the one who actually possesses the substantive right being asserted and has a legal right to enforce the claim. So if I was being sued by a debt collector and they have not shown me a copy of the Assignment, I would file a Motion to Dismiss or most definitely put in my Affirmative Defenses – “failed to name the real party in interest”.

That means they have not proven to the court or me that they have the right to sue me. Show me the paper work that gives you the right to sue me for this debt. Look up your local court rules and see if it is required to have the Assignment attached with the Complaint. If not, there are grounds here for a Motion to Dismiss for failure to comply and for failure to name the real party in interest.