This is a question for the sake of a story, I'm not seeking legal advice to use in court, just fact checking general laws. Any elaboration on interesting facets that may be useful for story writing is always welcomed as well :).
Let's say that there is an individual whose actual identity is not known: he goes by the alias Kal-el. Another individual, Clark Kent (yeah, I decided to get geeky with my names), has been accused of being this person in the past, and has told everyone that he is not this person, but he does know this person, and his real identity, but won't reveal that identity.
Later Kal-el is somehow related to a crime (accused of doing it, or a witness, whatever), and the police need to know who he is as part of their investigation, thus they talk to Clark. Let's say that Kal-el is a different person, but Clark does not want to admit this.
Could Clark ever be put in a position where he is forced to admit Kal-el's actual identity? I assume he usually wouldn't, but if he is aware of Kal-el's involvement in a crime at some point he becomes an accessory by not revealing it?
If there is a point at which Clark would usually be compelled by the court to give Kal-el's actual identity could Clark refuse to do it on the grounds that he has been accused of being Kal-el? Arguing that if he is Kal-el he would be bearing witness against himself to admit it and is thus protected by the fifth amendment. He has previously claimed he isn't in an unofficial capacity, but he is not going to make that claim legally, instead refusing to admit rather he is Kal-el or if Kal-el is someone who he knows.
If you only allow him to plead the fifth if he is Kal-el then the act of pleading the fifth still bears witness by implicitly stating he must actually be Kal-el, thus the only way to protect him from having to reveal the truth is by allowing him to plead the fifth even if he isn't actually Kal-el without this being considered lying to the court. Obviously you therefore can't force him to reveal the actual identity of Kal-el without implicitly forcing him to bear witness against himself as to whether or not he is this individual. Does this argument work?