The Grandmaster is a Marvel Comics character and antagonist in most stories depicting him. He is an ancient alien being who is morally ambiguous who finds enjoyment in challenging superheroes and pit them against each other. He is the brother of the Collector.
He is shown to own the space station Conjunction. In the episode "Space Cowboys", the Grandmaster and the Collector are revealed to be brothers, just as in Ultimate Spider-Man, both trying to kill each other. After the Collector tries to kill the Grandmaster with Moombas that explode if they eat anything other then grass the Grandmaster hires the Ravagers to bring the Moombas to Knowhere.
In the Collector's Collection, there is a portrait of both him and the Collector together. The two seem to be playing a game, with the Grandmaster throwing the game pieces from the table, seeming to show an extremely competitive trait to him.