From Halopedia, the Halo wiki
Good Samaritan is an Achievement in Halo 3: ODST. It is awarded for not killing any Engineers during a full campaign run through, in single player or Co-op. It is worth 20 Gamerscore. This achievement will unlock the full ODST Ballistic Battle Armor for the player's Xbox LIVE avatar on Halo Waypoint.
It is represented by an ODST helmet looking upwards shown in a white light.