What makes Malvolio a foil for Viola?

Malvolio could very well be a foil for Viola. He is supposedly good at his job, but lacks the good-natured empathy that Viola possesses. He rebukes Sir Toby and company very harshly, for which they resolve to get revenge. They play on his vanity and his pride by convincing him that Olivia loves him, and getting him to act foolish in front of her. He is finally locked up in a dark place, and tormented by Feste; in the last act, he comes forward and tells of his ordeal, and swears revenge on everyone involved, having not learned any lessons.