Glad you liked the Darkling! (But I have to say, I think YA is loaded with complex characters— Melina Marchetta’s Froi, Elizabeth Wein’s Queenie, and Holly Black’s Lila just came right to mind, but there are plenty more.)
Fictional influences for the Darkling… Flagg, Raistlin, David Bowie’s Goblin King, Kiefer Sutherland as David in Lost Boys, maybe a little Lestat in there? He’s pretty much every dark prince, rogue wizard, or dangerous sort I was ever drawn to and wanted more of.