A True Hero is a protagonist who is the true hero of the film, comic book, television show, or book. It could a hero that was revealed to be the real hero towards either the end or during the course of the film, or someone believed to be a villains, but eventfully was revealed as the true hero all along. SPOILERS ARE ALERT
Bender was the true hero of Slade Strikes Back as Joker was the true villlain of that story and much of the focus went on Joker and Bender's rivalry. It also a bit implied that he is the actual main protagonist of the M.O.D.A.B Series as opposed to Dib or Lizbeth.
Lizbeth in The Great Time Travel Adventure and the Beginning to some extent. As she was the moral chain of the team and without her involvement Slade would have won.
Ocelot may have been a villain BUT, in a way he was the true hero of the Metal Gear Solid series
The Grinch was a true hero in Christmas Bizzaro. He relised what true meaning of christmas is, but turned his back on Bizarro League.
isabella in LOTM: Next Gen Island Tour due to how Act 3 and 4 gave her own storyline with Team Free Will which led to them joining the heroes. Then Act 5 and Act 6 gave some more focus which had her work with the team closely before Act 7 revealed her as the one to stop the Niburu which she does after stopping Shaw, CSM, Wesker, and Blackgurumon.