I haven't seen anyone point this out yet
So I think everyone can agree that Ralsei hides somthing from us, his true nature for example. The thing I find strange about him is that he only speaks about the legend during the first chapter, which is weird because the info he gives us in chapter two would be really important for a "classic" prophecy, he is very elusive on how the world would end, not talking about the titans or the fact that darkners would all die. He reveals the roaring only in chapter two to prevent berdly from oppening a new fountain but even the queen doesn,'t know about that (though she is probably telling the truth about not knowing everything).
He also title himself prince of the dark world in the closet, even though being a prince with no subjects or king or queen to be a descendant of is kinda weird
My theory is that Ralsei made up/modified the legend and the roaring for his own interest (possibly to entertain the player and give them a goal in chapter one, and to stop what is a real threat to the world in chapter two)
Also, in the "the legend" cinematic, ralsei's name seems out of place, being to long (the legend could say "a human, a monster and a darkner/prince/dark prince" or something like that). His title even takes two lines of text like if he added himself into the picture
(But it also just be that toby expanded on the roaring after chapter one was out and rectified it in chapter two)
Anyway, ralsei has alway been sketchy imo, after the Spamton neo fight, he seemingly floats/goes into a wall (though it could very much be the perspective being weird)
And he closes the doors very brutally when leaving castle town in chapter one (but I think i'm just being paranoid)
Edit : I just thought about it but Ralsei is the only main being we encounter in the dark worlds who doesn't have his own theme (in contrast to lightners such as Suzie, Noelle and Berdly or darkners like King, Queen, Jevil, Spamton, Lancer, sweet caps and cake, Rouxls). Even K.round has their own theme but not Ralsei, one of the main members of our party and supposed hero of the legend. anyway it's a little deviation from my theory but I thought I'd mention it here
Edit : I didn't really give conclusion to my theory so I'm gonna say it now, I think that Ralsei was added to the universe of deltarune/the game and he isn't supposed to be here, thus the characters often forget him/don't aknowledge him (queen for example). You could imagine he's here because of gaster or something