After running a BG3 run with my donkeyboy, I think I at last settled on who he is, his appearance, and his backstory.

Raised in privilege, but his governess was a hag. He was a trained duelist and got into criminal trouble a lot. Mummy & daddy paid his way out. Until he stole from the wrong person, the hag who raised him. She did nearly grow fond of him before his mother’s magic discovered the Hag, so to vex him, she gave him the donkey features. The more he appeases this new patron, he will just be stuck with the ears & tail. The less he appeases, he will become more of the jackass he is inside. Sassy, spoiled, and insecure, he reluctantly has to go on adventures to keep himself, well… himself. Not a lone wolf though, he prefers a barbarian get the kills done and the cleric heal his wounds. He can duel and call his pact, a rogue-warlock swashbuckler-archfey. Fenwick is (finally) I think a character I love.