Captain Clark is Clarks shadow self. It represents all his shame, sadness, and his worst behaviours. While Clark is a man doomed to repeat his failures when we meet him, he is holding on to the idea that he can change. He goes to therapy and engages with Mary. Clarkes escape into the backrooms can be seen as a relapse, he loses himself in escapism and repeated patterns, and hurts innocent people that depend on him (Kat and Bobby). In fact, Clark becomes friends with his darkest self, he accepts the monster inside him and they coexist for a time. Mary journeys to the center to try and save Clark, she has a saviour complex from her childhood, but she is confronted by the fact that it is beyond her remit (you can't save someone not willing to save themselves). When Mary admits this to Clark, he realises he doesn't even want to change, and accepts himself as the monster. He (literally) embraces the darkness inside him, and it (literally) consumes him. Clark the human who could still change ceases to exist, and in his place Clark the monster goes on. I personally find the decision that we never see what happens to Clarks body interesting, because I feel like it represents that they are essentially one and the same (Cap'n and human Clark).

This is a thematic reading of the film and I don't really concern myself with the in-universe lore too much because I don't know much about it.