From Halopedia, the Halo wiki
Retrieved from "https://www.halopedia.org/index.php?title=Hero_(medal)&oldid=1038567"
The Hero medal is awarded in Halo 3: ODST and Halo: Reach Firefight mode for being the only player left alive, and surviving a wave. Upon the next wave, your team is revived and you are awarded with the Hero medal.
Originally, According to the Bungie Day (7/7/09) Podcast, the last man standing would be able to revive their entire team by earning three hero medals. However this feature was removed in the final release of the game.