To unlock this achivement, you must revive another player in the game world. This will require you to play co-op split screen with another person, or invite another player to your game online. You should then wait for them to die (a not so unlikely event in this game to begin with) and then use Interact to revive them.
Alternatively, if you have another computer or a controller, you can create another character on your own while you’re in game. Just bring up the menu and select ‘Start Split’. Once you’ve created the character, control them and ensure their defeat by jumping from a cliff or throwing them into some mobs. You can now use your main character to revive them.