Fagin and Nancy
What was the deal with their relationship? When Sykes was being cruel to Nancy, Fagin sort of looked sorry for her, but then he agrees with Sykes when he spurs venom about her. Then he speaks kindly to Nancy to try and figure out what she's up to. Then he sends the Artful Dodger to spy on her, and when he finds out she's talked to someone about Oliver, he's then forced to tell Sykes about it, which sends him in a rage, but Fagin pleads with him not to be violent with Nancy.
I had a feeling maybe Fagin is scared of Sykes in some ways or another, sometimes he stands up to him, and other times he doesn't.