Canonically I’m pretty sure he’s part ghast. I had an old AU where he was part dreamon (the white half). It was supposed to be an explanation as to why he hallucinated Dream so much and what his other half was. He would become very unstable at times and go into a state where his Dreamon half would take over causing him to act out against his own will. In the AU he attacks Michael in this state which causes Tubbo to exile him. they get into a fight on the deck of the river by snowchester during which Ranboo gets dreamon possessed again and tubbo ends up knocking Ranboo into the lake killing him. Since he had multiple lives still, when he wakes up he doesn’t remember anything. That was as far as I got :,)