Foils are two characters whose attributes enhance one another's characterization through their contrast. In Look Back in Anger, Jimmy and Cliff are foils. Jimmy is fiery and intelligent. He's also cruel, especially to his wife, Alison, and to Cliff. He makes excuses for his verbal attacks on Alison, claiming that she's too possessive and that she doesn't understand him because she hasn't suffered like he has. He is self-centered and unable to identify with others, as shown by his treatment of Alison when they lose their baby and the fact that he cheats on her with Helena.
Cliff, meanwhile, is sympathetic and kind. He may not be as well-read as Jimmy, but what he lacks in smarts, he makes up for in compassion and understanding. He tries to defend Alison from Jimmy's abuse, and when she burns her arm, it's Cliff who bandages her up. Cliff's appreciation of, and care for, Alison highlights how badly Jimmy treats her.