Collector

"One down, five to go."

- His status after keeping the Aether

Taneleer Tivan aka The Collector is an alien and an obsessive keeper of the largest collection of interstellar fauna, relics and species in the galaxy. The Collector operates out of a place in space aptly named Knowhere. He appears in a mid-credits scene in 2013's Thor: The Dark World, where he accepts the Aether from Volstagg and Sif, under the pretense that it needed to be protected.

After they leave, however, it becomes evident that he had ulterior motives for accepting the Aether, planning to use it and find five other objects, presumably including the Tesseract. The Collector will be more prominent in 2014's Guardians of the Galaxy. When he encounters Peter Quill and his band, The Collector attempts to strike a dubious bargain.

Trivia

 * In Disney INFINITY, The Hoarder from the The Incredibles Playset is inspired by The Collector.
 * Cosmo from the Guardians of the Galaxy comic is a part of the Collector's collection.