Dunbar Descripition

William arrived at Kadic Academy in New Order because he was kicked out of his old school for pasting love letters everywhere. He enrolled in the ninth grade, quickly taking a liking to Yumi. However, due to his jerky attitude and constant need to outdo Ulrich constantly, Yumi liked him less and less, although it is still implied he has a crush on her.
William's personality is self-centered and overconfident but still caring. It is often indicated most things he does are done to improve his social status. He constantly strives to outdo others in whatever they seem to be doing. Thus, William could be treated as the opposite of Ulrich, whom he doesn't seem to think of as that large of a threat to his fancy and pursuit of Yumi in romantic fashion. William, due to his brash nature, might sometimes harm others without meaning to. For example, in Final Round, he challenged the Scyphozoa to a battle, and lost, only to trying prove himself a good Lyoko Warrior. William might suffer from inner low self-confidence, but if this is so, he constantly wants to regret this and best his schoolmates. However, he does act more mature in certain situations. For example, in Attack of the Zombies, William did all he could to protect the remaining students from being zombified and wouldn't let anyone leave for their own safety, though in doing this he delayed the group from getting to Lyoko but he meant well.

XANA-William as a spectre.

In The Secret, William helps the Lyoko Warriors tremendously in their fight against XANA, doing everything except going to Lyoko himself. After that, Team Lyoko held a vote about whether or not William should be allowed to become a Lyoko Warrior. (In order for him to get in, it had to be a unanimous vote.) However, Yumi voted against him joining because she felt that he could not be trusted, and he supposedly forgot all about it after a Time reversion. However, it seems that he began to dream about the incident afterwards, as he revealed later in the episode The Secret.
Yumi still does not think that he can be trusted in Double Trouble, but she changes her mind by the end of the episode after he helps out once again, and asks him to join their group. He accepts, and, in Final Round, is virtualized onto Lyoko.
On Lyoko, William is dressed in silver-black armor with a red belt. His weapon is a large sword which can fire energy beams that act like razors, although because he first exhibited this ability while possessed by XANA, it might be that this was XANA-induced, and not actually a genuine power. His sword is quite heavy, although it is powerful, and it can cut monsters in half quite easily (or at least, cut Creepers in half easily).
Like Odd, William seems to enjoy fighting monsters on Lyoko more often then not, perhaps a bit too much... On his very first mission to Lyoko in Final Round, his desire to keep fighting monsters led him to pick a fight with the Scyphozoa all by himself, although Jeremie repeatedly told him to run away. This led to William losing and being possessed by XANA, who used him to completely destroy Lyoko by destroying The Core.

William returns to Earth in the episode Down to Earth.

In the episode The Lake, William faces off against Yumi on Lyoko. He initially gains an advantage, and is about to devirtualize her when she desperately reminds him that he is a Lyoko Warrior. He reacts, having flashbacks to when he was initiated into the group, similar to the way that Aelita had memories of her past. He seems to momentarily break free of XANA's control, but eventually succumbs to his influence again. This is the first instance of one of XANA's possessed slaves exhibiting free will, if only a bit. It also shows that William is not following XANA willingly.
William is known to be able to move swiftly through the Network easily, but XANA made him his own personal Nav Skid, the Rorkal, which does not attach to the Skidbladnir. He needs it in order to attack Team Lyoko while in the Network, because without it all he can do is just pass through it; he can't affect it like Franz Hopper can.
William is finally set free in Down to Earth thanks to a program Jeremie created. The program required William to be devirtualized so it could activate. Thanks to Odd, that happened. Afterwards he replaced his clone at Kadic and his affection for Yumi resurfaced. In Fight to the Finish he wants to gain the group's trust back. However, all but Aelita are wary of trusting him again. So he remains in the lab with Jeremie. This, however, turned out to be a huge mistake as XANA takes this chance to posses William (again) and fight off Yumi and Ulrich. However, this possession was very short as XANA was destroyed, returning William back to his old self.
He later returns back to his old ways of flirting with Yumi. Although he is seemingly turned down by her in Echoes, it is left uncertain if he is still allies with the members of Team Lyoko. William did not join them in the shutting down of the supercomputer