Which characters count as "additional" towards the One for All Trophy trophy?
With any of the additional characters, perform 20 ‘Revive’ actions.
Only the three characters provided in The Battle Hardened Pack DLC (which is the DLC which adds the trophy) count towards this trophy: The Governor, The Highlander, and The Wild Coyote Man. Since they aren't labeled when selecting characters, here's an image of them:
The only mode out there that lets you choose one of them every time is Wolfpack mode. Playing in Manhunt or other team modes requires the team leader to select one of these three characters, which can get a bit frustrating. If you play Wolfpack with other players, you can always just let a teammate go attack a target and likely there will be others there that will stun him and run off, then you can swoop in for the revive.