I'm stuck on Red being Katarina because my intuition tells me this is it. It's really wacky, but it also works. I have yet to see another character come up where I'm like "Nope. That's actually Katarina." Red has been saying things for the past few seasons or so that seem like things only a mother would say, especially when he's talked about his childhood. I can't remember those scenes word-for-word, but that's how I got stuck on the "Red is Katarina" theory. I haven't budged since.