It was earlier reported that he would be playing Ronan the Accuser, the Kree Empire's juror and military governor in space, but that information proved to be false.
Here is Marvel's rundown on John C. Reilly's Rhomann Dey.
"Rhomann Dey was the leader of the Nova Corps on the planet Xandar when the planet was destroyed by the space pirate Zorr. Seemingly everyone else was killed in the attack, including Dey's family. Dey swore vengeance on Zorr and chased him across the galaxy, battling him along the way. He became mortally wounded and paralyzed after an encounter causing him to seek a replacement who could stop Zorr and make him pay for his crimes. His ship came into orbit of Earth, where Zorr was taking refuge, and he selected a human to inherit his power. That human was a young Richard Rider.
Rhomann Dey linked minds with Rider and explained to him the new powers he has received as well as the threat of Zorr on his planet. At first, Rider thought the encounter was a dream, but soon discovered his powers and accepted his role as Nova. While out testing them for the first time he ran into Zorr who was on a rampage in New York City. Nova decided to fulfill the one who gave him his powers dying wish and defeat Zorr. During the battle, Zorr was teleported away by Rhomann Dey himself. Dey killed Zorr before he himself died, ready to join his family and friends in the afterlife."