From Halopedia, the Halo wiki
Hero is an achievement in Halo 2 for Windows Vista and Halo: The Master Chief Collection. It is awarded for completing the campaign on Heroic difficulty. It is worth 50 Gamerscore in the PC game; in The Master Chief Collection it is only worth 20 Gamerscore.
In the PC game, it is represented by the symbol of the Heroic difficulty; two Combat Knives crossed over a shield. In The Master Chief Collection it is represented by a screenshot of the crowd at Thel 'Vadam's shaming.