I know a popular theory is that Reddington is actually Katarina who became him in order to protect Elizabeth. I never liked this theory and while rewatching the show saw something I think rather convincingly proves it wrong.

In season 6 episode 18, after Red learns that Elizabeth turned him into the police and Dembe kept the secret, he goes to Dom to think. While there he asks why Dom gave up everything after Katarina betrayed him. He pressures Dom by saying if it was Katarina asking not himself, Red, what would he say to her. Dom gets angry and says it doesn’t matter because she isn’t the one asking. Dom also almost always treats Red with resentment.

I get that the resentment and anger could be because Katarina who is now Red betrayed his country, but the interactions between the two always talking about Katarina in third person make that unlikely in my opinion. And because it seems very apparent that Dom does know Reddington’s true identity, does this not show that Red is indeed not Katarina? And to me, a close friend of Katarina’s from Russia who took Red’s identity as a way to protect his friend’s daughter when she no longer could.