The Goetic spirits, for the most part, have appeared as very beautiful females of diverse cultures. I have yet to meet Andromalius though. Satan/Lucifer was a guy in a suit. Belial appeared to me, looking similar to Jared Leto below. “A slick lawyer with slick-black hair in a suit.” King Paimon looks like a clean-shaven version of the Dos Equis Beer guy.
The spirits use your representation or understanding of reality to appear and interact with you.
And Duchess Bune has appeared to me like the woman in the photo below. When I first saw this photo, I was shocked. This is exactly how I saw her! Very beautiful, Egyptian (Middle-Eastern) looking but has dimples cheeks, long black hair, slender body, sometimes nude, always barefoot… She’s always smiling and exudes happy energy. and like @AradiaX stated, she can also be “bratty” on occasion.