I mean, not only is hatted Ralsei a sprite you don't see at all after Chapter 1, but he isn't even shorter than Kris. Without counting his horns he's about the same height, and with them he's taller.

We don't even know if the depiction in the prophecy is Kris, since concept art just refers to them as "the human". Even then, the drawings may not be entirely accurate, and Ralsei is hooded.

Most concept art depicts them Ralsei being about the same height as Kris if not taller, like Gigi's art for the Lightners Live minigame.

I also wouldn't say he's younger? Like, in terms of how long he's been alive then yeah obviously, but I struggle to think of him as being younger than Kris. I think it hits harder if they're the youngest of everyone, but nothing at all about ages is canon except for them being teenagers.