A unique Outsider specifically trained for, as his name implies, infiltration and subterfuge. Hopefully, XCOM's base of operations remains secure...
As the last part of the intro/tutorial Chapter, this segment familiarizes you with Agent Selection before sending you to intercept him. He carries an M14 rifle, and can spawn Sectoids to assist him in battle. he's the first enemy to have shields, so lay onto him with focused fire to neutralize him.
After he is captured, a Mosaic beacon is extracted from his head, and he becomes a regular source of information throughout the game, coerced into telling details and history of Mosaic and the Zudjari Empire.
As the boss of The Last War mission, he retains his shielding, but now carries a Plasma Assault Cannon, teleportation, and the ability to make duplicates of himself.