Antagonist
The antagonist is the character who opposes the protagonist, but does not necessarily have to be villainous or evil.
The antagonist in Pygmalion is certainly not malicious because it is Eliza Doolittle. She rebels against Higgins for the poor way he treats everyone, and she makes him realize that a book cannot be judged by its cover. At the end of the play, she acquires certain manners that far surpass those of Higgins.