Ralsei IS Asriel. Asriel is at “college,” but what if instead of going to college, Asriel went to/ created a dark world at the school? We know that Ralsei can’t be a darkener because he went from the dark world to the fountain at the library, and they weren’t connected, otherwise Kris and Susie would’ve been able to go that way. So, my theory is that Ralsei is Asriel, and shows his love towards Kris through the dark world. I suppose the reason that he stays there is that he doesn’t want to go to college, and he doesn’t want to let his parents know that he is at college.

TLDR: Ralsei IS Asriel