That's what I was thinking. Maybe the Rood he actually knows is gone and just the traits left over and the body itself is what is left of Rood and the King is filling the gap?
That might explain why Rood has no memory of his past. The thing is...the personality is the same and there does seem to be two entities in Rood's body. Rood himself and then the King with the long dark hair which is what (and why) Rood changes so much physically when he Awakes.
So many questions.
I wonder if the next chapter will show us what happens after moment of 'Chapter 0' and if we find out how Kiel started Opion and how Rood became a magician and everything.
Not to mention how different Kiel is with Rood compared to how gentle he was with the first Rood. Should we call them Rood 1 and Rood 2?
Well actually, we can't really know if they have the same personality. Little Rood is sweet and docile, while big Rood is rather wild and rebellious. I thought it's just part of growing up, nothing quite odd. But it can also imply he's a completely different person. Little Rood's character hasn't been explored quite thoroughly so idk.
But I think I dib more that there are two entities within his body.

Spoilers....
So, no wonder Rood doesn't remember. He actually died.
Then somehow was brought back. But I don't think it's just Rood. I bet the King came back with Rood because that was the only way to bring him back. But why did the King do that? Did the King want out too? Or was the King moved by Kiel's tears and love for his brother?
So is our Rood actually Rood, a melding of Rood and the King, or the King with Rood's personality?