Thanos

"You will have your war, Asgardian. If you fail, if the Tesseract is kept from us, there will be no realm, no barren moon, no crevice where he can't find you. You think you know pain? He will make you long for something as sweet as pain."

- The Other describing Thanos' wrath

Thanos is a powerful alien and a warlord who rules over a dead dimension and commands an army of Chitauri. He allied himself with Loki to invade Earth.

Helping Loki
Thanos' past is unknown at this point and how he came to power. Thanos allied himself with the exiled Loki, providing him with the deadly army of the Chitauri, who will help him conquering Earth, in exchange for the Tesseract, a powerful energy source.

Once Loki had arrived on Earth, Thanos is contacted by him and The Other threatens the Asgardian god of mischief, saying that if he fails in defeating the Earthlings, Thanos will make him know a new kind of suffering.

After Loki was defeated, the Other reports to his master that the Earth's inhabitants are not as weak as they were supposed to be. He also declares that challenging the Earth's heroes would be to court Death. To this, Thanos simply smiles.

Powers and Abilities
Thanos' powers are yet unknown at this point. However, it is conceivable that he is extremely powerful, able to strike fear into an Asgardian god like Loki and command the Chitauri army unchallenged.

Relationships

 * Loki Laufeyson - Ally
 * The Other - Subordinate
 * Chitauri - Subordinates
 * Leviathans - Subordinates
 * The Avengers - Enemies

Behind the Scenes

 * Thanos' presence in the movie was kept top secret until the release of The Avengers.
 * Damion Poitier was credited as "Man 1" despite actually playing Thanos.
 * Kevin Feige has said that Thanos will likely be played by a different actor in future films.

Trivia

 * In the comics, Thanos has no relationships with Loki Laufeyson nor the Chitauri, but he's nevertheless one of the Avengers most historical foes.
 * In the comics, Thanos is the master of the Infinity Gauntlet, which he used in the eponymous series in an attempt to court Death.