I think he may just be a powerful Tiefling, who draws very heavily on its fiendish, devil side. In Forgotten Realms lore, Tieflings can grow wings by unlocking more of their infernal heritage. Potentially a very powerful warlock noble type character whose patron could assist the party?
Anyway, I'm excited to find out!
I have a feeling he may be related to Wyll, the origin character who is a Warlock. They might be rivals, each serving the same patron but fighting for their favor or serving different masters each with a different motive and trying to outmaneuver each other. We know that Wyll's patron is in a bit of trouble giving him leverage over them.
"Noble by birth, Wyll made his name as the heroic 'Blade of Frontiers'. He keeps his pact with a devil well-hidden, and is desperate to escape the hellish bargain - even if that means rescuing the seductive creature that made the deal."
Considering that BG3 takes place post Descent into Avernus, we know that the first layer of Hell is in *shudder* chaotic state and the various movers and shakers of the lower planes will be eager to capitalize on the opportunity.