So, Kubo's One-Shot BURN THE WITCH confirmed to take place in the same universe. And it introduce Dragons as another intelligence supernatural being, which has many species, with many shapes, and can mutate into another forms, etc...
And so, my theory is, Lille Barro is a kind of Disguiser Dragon, who take a Quincy body, and also, given power from Yhwach..
Lille is a Quincy, the first and foremost granted a Schrift, and a pure-blooded one at that - if he wasn't, he would've died in the first Auswählen.
Lille's eldritch-looking Quincy: Vollständig is a reference to the early depictions/descriptions of angels in the Bible and other such religious works as grotesque/eerie/unfathomable beings instead of winged humanoids.
I can't see Kubo making a Dragon that looks like THAT.