William Dunbar is the secondary tetartagonist from Code Lyoko. joined Code Lyoko in the second season as one of Yumi's classmates. In Final Round, he became the sixth member of Lyoko Warriors, until his first trip into Lyoko, when X.A.N.A. possessed him and used him as his general until episode Down to Earth where he was freed and returned as an ally. He then returns in Code Lyoko Evolution, and permanently joins the team at the end of the episode Rivalry.
William was kind of a loner at first, having been kicked out of his previous school for posting love letters everywhere, but Yumi soon takes to him as a friend. To nobody's surprise, William wants to be more than just friends, causing conflict between him and Ulrich when both of them vie for Yumi's affections. His appearances in the show become more frequent during the third season, when Jeremie suggests to the team that William join their group, much to the disagreement of the rest of the warriors. As fighting on Lyoko becomes harder and harder and X.A.N.A. starts to force Aelita to delete several of Lyoko's sectors, the group decides to let William join.
However, X.A.N.A. takes advantage of William's ineptitude and uses the Scyphozoa to take control of his mind and body, effectively trapping him in Lyoko. Jeremie creates a stable polymorphic specter with William's basic memories, and a basic personality in order to prevent suspicion amongst the school. Over time, this "clone" becomes much more popular than William was, but due to his 'dumb' personality, he unknowingly causes trouble for our warriors. His personality also became a bit of a gag in the show, best seen in the episode Wreck Room. In episode Down to Earth, William is finally freed from X.A.N.A.'s control, and returns to Earth. However, his behavior pre-X.A.N.A. causes tension between him and the rest of the warriors, who are reluctant to associate with him further, as seen in Fight to the Finish.
In tandem with this, in Code Lyoko: Quest for Infinity, he was a secondary antagonist. However, he was still rescued from X.A.N.A., complete with recycled footage from "Down to Earth."
William's role in the new series, Code Lyoko Evolution, is to seek redemption, which he later succeeds. In Cortex, after Odd got himself locked up in utility room, William, instead of helping him out, went to Lyoko and takes Odd's place. For this, Odd was really mad at him, even having a thought of devirtualizing him. After rejoining the team, they stop fighting and became friendly to each other. In Rivalry, William and Ulrich puts down their disagreements after William saves Ulrich from falling into Digital Sea. William becomes more serious about the missions in order to be worthy for the others to trust.
When the Lyoko Warriors discover the existence of Source Codes, William appears to be the only one to not having them. This makes him a more distanced member, as he is the only Lyoko Warrior who cannot deactivate towers. That doesn't stop him from becoming vital support to the group when X.A.N.A. attacks or while exploring the Cortex, always fighting with passion and resolving many situations.
Being one of the codeless only deprives him of the ability to deactivate towers on Lyoko. On Earth, William is not targeted by spectres. This makes him the only one who can be exposed to them without risk, or almost none.
In Mutiny, William return in X.A.N.A.'s possession after he falls victim to the Scyphozoa. The monster failing to seize him completely, it remains attached to William. Later, he is freed thanks to his friends help.
William is not given a major role in the group's major decisions. He supports his friends in the fight against X.A.N.A. and its spectres, he supports them in the discovery of the Cortex and Tyron's Ninjas. Finally, he is on their side for all major decisions regarding what to do next, especially after the discovery of Aelita's mother's survival.
After the team infect the Cortex with their virus, William stands beside Aelita and the rest of the group when they go down to shut the Supercomputer back down again, a clear sign that he earned his place in the group... once and for all.