Isn't it perfect that the three kids meant to save the world, the ones meant to follow some prophecy set in glass, the ones who are starting to actively defy said prophecy, are gender-nonconforming?
Ralsei is referred to as exclusively he/him (correct me if there are other pronouns used for him) in-game, yet he is very emotional and cares about having a "cute" appearance.
Susie is "the girl", but she was brought up in such a way that would cause her to be rough and physically intimidating. She also doesn't like ribbons, which Ralsei implies she's supposed to wear (according to the prophecy).
Kris is non-binary. I will not be accepting "up to player interperetation" if not only EVERYONE ELSE in the game refers to them in this way, but Toby Fox is making it EXPLICIT that projecting onto Kris is pointless. Because they are their own person. But their entire existence, according to the prophecy, is reduced to "the cage, with human soul and parts". Not even human in the prophecy's eyes, just...with the same parts. Kris defies the prophecy and the concept of "blank-slate" main characters by pursuing inherently human things that they like. Music, art; creativity. Showing to no one in particular that they are a person.
These "boys-or-girls" are covered to the brim with narrative defiance and symbolism.
The prophecy expects these kids to do certain things, to be certain things.
But because, not despite of who they are, the world will balance again.
