Collector

"One down, five to go."

- The Collector after keeping the Aether

Taneleer Tivan (a.k.a. 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.

The Collector was created by Stan Lee and Don Heck.

Thor: The Dark World
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.

Guardians of the Galaxy
Having got the Aether, the Collector hired Gamora, the adoptive daughter of Thanos to get the Orb for him. However, the Collector also hired the Broker, preventing a possible failure of Gamora. Gamora kept her promise and brought the gem just as she promised. Arriving at the Museum, Gamora and her partners were welcomed by the Collector's assistant, Carina, who guided them with her master. The Collector was fascinated when he saw Groot. He then asked Groot if he could dispose of his body at the moment of his death (This conversation was not completed because they were interrupted by Rocket).

The Collector told the story of the Infinity Stones and as ancient beings used them to destroy entire civilizations like wheat in a field. A tired Rocket asked him to stop and pay them at once, The Collector was preparing the pay when he saw that Carina was approaching to the gem. He asked her to get away, she instead decided to touch the stone in a desesperate attempt to free the Collector but the power of the Orb is too big and Carina dies. Tivan was seriously injured after the explosion. In a post-credits scene shows the Collector having survived the explosion and drink with Howard the Duck.

Trivia

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