Who is the antagonist in the novel, The Ransom Note Blues?

Mrs. LaViolet is the novel's antagonist... the thief.

Alec and Gina beamed. Though Alec was feel¬ ing a little bit sad inside. He suspected Gina was too. Mrs. LaViolet seemed like she was mainly a nice person. Just a little bit . . . well . . . nuts to think she could steal a painting and get away with it.