The term 'VILLAIN'S ADVERSARY' can be solved by the crossword solution 'HERO', which is a four-letter word. A HERO is a person who is admired for their courage, outstanding achievements, or noble qualities. Heroes are often seen as the protagonists in stories, and they are usually pitted against a villain, who is the antagonist. Therefore, a HERO can be seen as the adversary of a villain, making the crossword solution 'HERO' an appropriate answer to the term 'VILLAIN'S ADVERSARY'.