>>717925239 (OP)
I always figured it was a kinda, shapeshifter, sorta succubus type thing.
It explicitly feeds on emotion, and there's regular mention of the red thing as the "core" of the line. Gardevoir wasn't in the Humanshape egg group until very recently, instead being in the Indeterminate egg group, where things like ghosts, slimes, and other things with amorphous bodies go, where it still is, it just gained Humanshape in like gen 8. For most of Pokemon it was Indeterminate only.
The appearance of the line is thus completely arbitrary, besides coloration maybe, and the presence of the red crystal, and is chosen specifically to inspire emotions to feed on. Additionally, Ralts usually specifically looks like a human child to garner protection by humans, and ingratiate into society, where it can have the greatest access to food
