I believe Reddington is the real Katarina Restova. Because:
1. Katarina had been shown as bisexual. So the now Reddington has no problem with the ladies in the sex department.
2. The now dead Katarina was a replacement to the real one. Being brain washed n programmed by Reddington to believe and have acted as Katarina. A decoy so that Townsend would continue chase. But all this while, Red hid Katarina in Paris until she turned up in season 6 alived n gutting Raymond for answers. This Katarina didnt a lot of things. Her memory was quite limited and only recent memories. Never a childhood memories. Never a mother daughter baby memories.
3. From Season 1 Episode 1, Reddington has promised Keen that he wouldnt ever lie to Keen n that he wasnt her father. He hasnt break that promise until now! In Season 4 finale, it was Liz Keen who made her own conclusion of the DNA result that Red was his father. Never ever have Reddington say that he was her father. In fact Reddington had told Liz the truth that she had shot n killed her own father on the night where Katarina went to the real Reddington home to get her Liz back. Everybody knew now that Reddington had Liz's brain brainwashed to conceal the truth she had killed her real father. The corpose was burnt n charred in the house fire of the aftermath the night Liz killed her father. The real Katarina now Reddington hated Liz's childhood home so badly Red had it burnt down in Season 1 (or 2).
4. In Season 5, the secret was out about plastic surgery, personality n voices change operations for bad people to change identity. I believe this was the very time n juncture that the real Katarina transformed into the now Reddington but not before have a replacement Katarina already on standby. Because the nurse who was involved with the surgery testified to FBI that a man and a woman had come to do the identity transformation operations.
5. The dead Decoy Katarina had no emotional memories n tendecies toward hee long lost father Dom, Liz her daughter, n late mother. Nothing. But the now Reddington does have those.
6. The time when Reddington told Liz abt N13 when Liz confronted him about him as the N13. Red answered, "... eerrrr it's complicated". Wanna know why? Because 'Reddington' was (is) the N13 but actually the real one is already dead, shot dead by his own daughter, Liz! And the now Reddington, though bodily n appearance wise is 'Reddington' for all intends n purposes! So he is N13 but at the same time he is not, because he is really KATARINA ROSTOVA the transgender Raymond Reddington now.
7. Red doesnt want Liz to know he is Liz's actual mother. He will take this secret to his grave. That's why he had no qualms shooting killed Decoy Katarina who had no motherly instinct toward Liz whatsoever. Except being her tools to get to Reddington previously.
8. That's why, despite whatever Liz has done to Red now, Red still bequeth all his assets n possesions for Liz n Agnes to inherit.
That's what mothers would do to their daughters, sons, n grandchildren. That bons. That love. Only mothers n fathers have. Forgiving their children, always. Reddington has all that towards Liz.
Anyway, it wouldnt be that all surprising to know later that Cooper is in fact the N13. Plot twist alternative.
In conclusion, I believe Liz Keen has got it all wrong about Raymond Reddington. And she would regret very2 much in the end.