Hello

The first character that comes to mind is Lymond from Dorothy Dunnett’s Lymond Chronicles but those books are DENSE reads (brilliant but like a damn thicket). I absolutely adore The Curse Workers Trilogy by Holly Black (magical mafia). The Female of the Species by Mindy McGuinness isn’t fantasy but has a quality female antihero (rape and abuse tw). The Lies of Locke Lamora is a fantasy heist book (series is Gentleman Bastards) and is plenty violent, but Locke is a very cheerful thief, so not really cast in the same mold as Kaz? It is also definitely not YA. Ohhh and The Queen’s Thief series by Megan Whalen Turner. Gen is absolutely nothing like Kaz, but what a brilliant schemer. I just adore him. 

Guys, help me out. Ruthless antihero sourpusses in Young Adult?