Who is the antagonist of sonata for harp and bicycle?

In "Sonata for Harp and Bicycle" by Joan Aiken, the antagonist is the character of Mr. R. M. B. (Mr. B), who represents the oppressive force of the insurance company and its bureaucratic nature. He embodies the conflict between the whimsical, imaginative aspects of life and the dull, rigid constraints of society. His attempts to enforce rules and suppress creativity ultimately clash with the protagonist’s desire for freedom and expression.