(Usually with the, frequently in plural) a good or virtuous person; especially one who is on the side of good against evil in a story, film, etc., and whom the reader or audience is intended to support or identify with; (in extended use) a person who is on one's side; frequently opposed to "bad guy".
Origin
1920s; earliest use found in Boys' Life.
Definition of good guy in US English:
good guy
noun
US informal
(Usually with the, frequently in plural) a good or virtuous person; especially one who is on the side of good against evil in a story, film, etc., and whom the reader or audience is intended to support or identify with; (in extended use) a person who is on one's side; frequently opposed to "bad guy".