adding to this because i can't stop thinking about it:
what if it's not that ralsei knows about the prophecy, but rather is aware of the time loop? or more specifically has been conscious of it the whole time? he knows the prophecy must come true because it HAS for him. over and over again. (i believe kris is also aware of the time loop to some degree. but that's a different tangent.)
this adds credence to my theory of what the "end" of the prophecy really is.
i believe that the end of the prophecy is one of the members of the prophecy turning against the others. but the prophecy doesn't specify WHO. what that means SPECIFICALLY, i don't know, but it's starting to look like it could mean dealing a sort of final blow to the world of deltarune, to the others, or the timeline, or whatever.
and little ralsei, not wanting his friends to have to deal with the grief of betraying his friends that he's experienced over and over again, basically volunteers to be the one to turn against them. it's a self fulfilling prophecy.
it's why he feels so guilty all the time. tries to be the nicest he can. wants his friends to experience the most joy possible and wants them to know that he really cares about them. because in the "end" he anticipates having to do something horrible. because, according to prophecy, someone has to do it.
this would give ralsei the same narrative use as asriel, albeit a sort of foil. a character who has been thru countless lifetimes over and over again. but instead of fighting back and getting angry like asriel/flowey, he leans into the learned helplessness. it's kill or be killed. asriel chose kill. ralsei chooses be killed. (or whatever that would look like for the ending of deltarune.)
if this is correct, in a way, ralsei and asriel would represent us, the player. utdr has always been about making choices, with both games having distinctly different messages: your choices matter (undertale) and your choices don't matter (deltarune.)
what choice will you make when the world wants has infinite choices, each leading to different endings? (will you exhaust every option, stomping on little people to get whatever you want?)
and what choice will you make when the world is constantly reminding you that no matter what you choose, it won't matter? (besides, according to toby, the prophecy, ralsei, etc. there's only "one ending" anyway. tho i consider that to be a BIT of a stretch.)
of course, choosing good even if you think it won't matter in the end is the best choice one could make. this is the true theme of undertale. and i believe it's the true theme of deltarune too. it's not the ending that matters. it's what you do to get there.
