Agent is a playable character in Ada's campaign ONLY if it in is co-op, he does not appear if the campaign is played in solo. He was put into the game from a patch that was released in December 2012. Before the patch Agent was already on disc and the only way at that time to access him was to mod the game. Besides being added to Ada's campaign, Agent is also available in all other extra modes (such as Mercenaries). Agent doesn't really have any history, as he is only really there to give Ada a partner in her campaign. He can not open doors or operate any puzzles, leaving the host (Ada player) having to do most things.
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