.Let's look for a moment at the meaning of ka-tet: One from Many. That can also apply to demons - ie Legion. One demon composed of many. And this is a name that is used in connection with just about all of King's evil magicians
Walter Paddick is actually the original form of Marten/Flagg (as we learn in The Dark Tower). He was born as a human to a miller in Delain. However, after running away from home at 13 he uses and abuses magic until he achieves his almost immortal, shape shifting status.It is possible (and I say even plausible) that as each of the wizards sent against Roland were defeated - their physical and mystical being was obsorbed by the next. So Marten is consumed/ possessed by Walter who is in turn consumed by Flagg. The ka's (or spirits) of each remaining in the single host body - Legion.
Marten Broadcloak, Walter O'Dim, Richard Fannin, Rudin Filaro, Randall Flagg and the Walkin' Dude are all aliases of Walter's. He can change his face aswell as his name, and can also travel through other worlds and time. He is often defeated, but never physically killed until he meets Mordred, so rather than being absorbed by a new entity, he simply shapeshifts and moves on.
(Reference: The Dark Tower Complete Concordance.)