December is one of the Observers. He appears to be very strict and appears to have some type of sway among the group, making it look like he is the leader of the scientist's division.
In 1985, August and December went to see the Alternate Universe's production of 'Back to the Future' (starring Eric Stoltz). September met up with them after the movie and told them that Walternate was accidentally distracted from finding a cure for Peter Bishop. This was a mistake of September's own making. August and December explained that there would be an opportunity for September to put the timeline back on track.("Olivia. In the Lab. With the Revolver")
He is first seen condemning his fellow observer August while eating with him. He forgives August for saving Christine Hollis as August loved her. He then contacts Donald Long to kill Christine. He later appears in carnival park where Olivia Dunham and Ella Blake are riding a roller coaster. He and September discuss how difficult things are about to get for Olivia. ("August")
September explained to December that he thought Walter could be persuaded to let Peter die. He doesn't believe Walter Bishop has changed but was corrected by September's actions. After September's manipulation, December agrees that when the time comes, Walter will be able to do it again. ("The Firefly")
The next time December is seen is right after Peter Bishop was erased from the timeline at the Statue of Liberty.("The Day We Died")
December met with September and tasked him to finalize Peter's erasure from the timeline. He handed him a device that would remove Peter from the timeline once and for all, explaining that no one could ever know Peter lived to be a man. ("Neither Here Nor There")
March and December discovered a device belonging to September. He was informed by March that Peter had returned to the timeline. ("Making Angels")
After finding out that September had not followed his orders, an angered December vowed to make him face the consequences of his actions upon resurfacing in the timeline. ("The End of All Things")
September seeks out December to help aid Walter's Plan. December, who is still an Observer, has the ability to travel to 2609 and gain a critical power source for the device. He does so because he owed September a favor. However, when Olivia and Astrid went to retrieve the piece, they discovered loyalists had infiltrated December's apartment. They found his bloodied body hanging from the ceiling, suggesting that his murder was set up to look like a suicide.