ok i might sound insane because i haven't finished ch.2 yet and i don't know proper lore all too well but has anyone considered that maybe Ralsei is an ideal version of Kris?
the game so far has this running theme of melancholic smalltown nostalgia. of Kris trying to navigate life in a town where everyone knows of them but doesn't know them, as well as reconciling with their split parents and missing Asriel. Not only missing Asriel but a sort of underlying comparison. we see Asriel's side of the room is colorful and littered with trophies and nearly everyone in town has something to say about how fun he was or how they miss him.
if we assume that Ralsei is partially made up of Kris' fake horns from childhood that Toriel mentions, maybe it can give credence to the idea that he IS a different version of Kris; One that Kris thinks would be more palatable to their hometown and their family? maybe even, secretly, to themself? a version of them that's outgoing and kind, polite and softspoken and who everyone likes. a version of them that looks a lot like...Asriel.
idk...it was just a thought i had during the scene where Susie reveals she taught Ralsei what sarcasm is. it made me think about how being sarcastic and mischievous is a big part of Kris' personality and how Ralsei seems to be the antithesis to all of that; the complete inverse of the weird, creepy, quiet kid. i feel like it would explain small things like why Noelle doesn't acknowledge Ralsei outside of when she first sees him and why when she's given Ralsei tea to heal she claims the cup is empty. Noelle and Kris have been neighbors their whole lives, a little closer than neighbors because of how friendly their fathers were (but never quite friends, until now) so she knows Kris fairly well or at least seems to think she knows them. so of course, if Ralsei is meant to be a projection of what Kris subconsciously thinks they should be, Noelle would see right through him.. like he's a ghost, or a bad imitation. Susie wouldn't feel the same because she's never been close to Kris until now, she doesn't know too much below the surface about them. it would also make Kris not getting healed very much by, and not having a reaction to, Ralsei tea when they're given it.
maybe it's really silly and there's something obvious i don't know yet or am missing entirely, but imo it would fit the theme of a seemingly happy little down struck down by a bunch of small tragedies and the kids of those tragedies figuring out how to grapple with them and their own adolescence.