1. "hero" can be male or female; "heroine" is female. "Protagonist" just means "the main character in the story" without assuming that the character is good. A person who opposes the protagonist is an antagonist. 2. When "performance" refers to acting, it is countable. I enjoyed Bob and Jill's performances in the play. I really enjoyed that play. I watched 3 different performances of it. (=I watched the play 3 times.) 3. When "performance" refers to "ability" or "delivered service", it is uncountable. These two computers don't have very good performance. Bob and Jill's performance on the task was excellent.

1)The opposite of villain in this context is usually hero or heroine, yes. "Hero/heroine" and "villain" are used as character moral archetypes for good and evil characters respectively. Since the subject in your sentence is female and plays villains, her opposite would be a heroine. You might hear in spoken English that a woman is playing the hero of a story (as opposed to a heroine), but if you want to err on the side of caution, go with the feminine form (heroine). 2) Performances are countable when they refer to the act of presenting roles, plays or concerts. An actor like Ray Liotta is known for his unforgettable performances in numerous gangster films. Performance is uncountable when it refers to the capability or measure of success of a process. Reducing the amount of working hours has been linked to improvements in work performance and an increase in overall happiness of the employees.