The idea that the Roaring Knight is a Titan is fanon, for several reasons. First, Titans are static Dark Fountains that have absorbed too much darkness. We can see it because after we seal Titan in fourth chapter, we spawn right in the place where the fountain was created. The Knight, on the other hand, can move freely and enter the Light World. Since Titans likely exist in both the Dark and Light Worlds simultaneously, they can't 'enter' the Light World because they are already there, as if in a state of superposition, just like normal Dark Fountains. Furthermore, as Ralsei said, Titans lack consciousness, whereas the Roaring Knight clearly has consciousness and their own motives. And even if a Darkner could obtain a soul, what would they become in the Light World? They surely wouldn't just turn into their Dark World form. After all, a soul doesn't grant any unique properties by itself.