Callie's the main one, obviously. She's our narrator and the Siren, this girl with a dark past who can enchant people with her voice, but mostly she's just trying to run her sketchy little favor business in the human world. Then you've got Desmond Flynn, the Bargainer himself. King of the Night, ruler of the fae realm of Somnia, and Callie's enigmatic... everything? Client, protector, love interest, source of eternal frustration. Their dynamic is the engine of the series.
But saying it's just about them feels reductive. There's Temper, Callie's best friend who's a witch and provides most of the comic relief and grounding. Malaki, Flynn's second-in-command and a serious broody type with his own complicated loyalties. A lot of the later books, especially 'Gilded Bones', really flesh out the wider circle of allies and threats, like the various fae courtiers and the mysterious antagonist pulling strings in the background.
Honestly, I think the most interesting 'character' sometimes is the history between Callie and Flynn—those seven years apart, the favors, the secrets. It's like a ghost in the room for the entire first book.