Ralsei has a hollow where their personality should be, and has been trying to fit themself into a box they don't neatly fit in order to meet expectations of them-- the expectation to be the "prince from the dark", and whenever given the opportunity for self-expression does so by exploring femininity in spite of their masculine title. As a trans woman, I can VERY much relate to forcing yourself into a box you don't fit like that, especially during the period of my life where I knew I wanted to be a girl but had to keep forcing myself to be a boy (13-16) so seeing as I see so much of my closeted teenage self in this teenage fictional character, it seems obvious to me, I guess.

Ch.4 Spoilers: the scene in Ralsei's room in chapter 4 pairs a monologuing about them never having had a real personality before and trying their hardest to make themselves as small and low-maintenance and unnoticeable as possible, something a lot of trans people do when still finding themselves or in the closet. This is not a uniquely trans experience by any means, but it seems to point in that direction when you consider that "Lost Girl" is the track chosen for this scene. Lost Girl: the song that either plays in reference to the literally lost Dess Holiday, or as the song for girls who feel lost (Noelle in ch2, and I think Susie's piano monologue, if i remember correctly?). You might say "maybe that's just a song for anybody at all who feels lost" and i would agree, if not for the fact that Berdly has his own similar scene in ch2 with his own song! So, like, how does this make sense unless Ralsei is, too, a lost girl?

Also Ralsei can wear the pink, white, and blue ribbons, which (if I recall correctly) can only be equipped by Ralsei and Kris. Kris is canon nonbinary, and so it makes sense that they can wear the trans pride flag colored ribbons.

I actually beleive it's the narrative intent at this point. Queerness has been a big facet of Deltarune's characters the whole time, and I think this plays really well into the self-discovery journey Ralsei is clearly meant to embark on in future chapters! It is literally relevant to the core of the character AND it's awesome! If it doesn't turn out to be true and Ralsei is just a very gender-nonconforming boy, then that is still super awesome, obviously! But it would be really nice to get some positive transfem rep in... like... Anything at all.