A Redditor has discovered a number of secret Mass Effect 3 characters for use in the game's multiplayer mode, which plays a lot like the Horde Mode in Gears of War 3.
The image he posted onto the website reveals cards for currently unplayable characters, which include the Geth Engineer, Geth Infiltrator, Batarian Soldier, Batarian Sentinel, Krogan Battlemaster, and the Asari Justicar.
Persumably, the characters can be unlocked through the acquisition of Veteran and Spectre cards, which provide the player with new weapons, classes, kits, and aesthetic unlocks for their characters. Players must buy these cards through in-game currency, which is procured from playing multiplayer missions. You can also buy the cards with real money for 1-2 dollars each.
These classes remain unconfirmed by BioWare, but the guy who posted the image claims that he discovered them when he downloaded a test DLC from the developer server.