William Dunbar joined Code Lyoko in the second season as one of Yumi's classmates. In "Final Round", he became the sixth member of Team Lyoko, 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.
In the season 5 appears in the episodes "Requiem" (where makes a cameo), "X.A.N.A. Is Stronger" (when he rejoins Lyoko-Warriors and returns to Lyoko) and "William vs William"). In this last episode fight against X.A.N.A-William (clone) and in the end William's parents move and he leaves Kadic. Later he makes a visit to Kadic in episode "William Visits Kadic" that Lyoko-Warriors have organized a party for him, but X.A.N.A. take control of his parents to take him to Lyoko and be re-possessed by the Scyphozoa, but fails and leave Kadic with their parents again.
- ID Card: The back of William's ID card havea triangle symbol, in another, and still in another triangle 10 times.
- Health: 100 Life Points
- Zweihänder - William wields a big sword called Zweihänder. Its name literally means "Two-Hander" in German. Traditionally, Zweihänders are wielded with two hands due to their great weight. Evidence of the sword's weight can be found in the episode Final Round, when William attempted to hold it with one hand, and then dropped it. His sword also fires energy blasts when he swings it.
- Lyoko Powers/Abilities:
- Super-smoke - An ability much like Ulrich's super-sprint, but instead of simply running at speed, William becomes smoke to pass away from people, monsters or things. The black smoke can suspend other people in a paralyzed state. He still has this power in season 5. Since he has no vehicle, this is his only transportation.
- Second View - The ability to see much farther away in the same sector. This is was only available while under control of X.A.N.A.
- Levitation - An ability that allows him to levitate by holding his Zweihänder by both hands and pointing downward. Only available under control of X.A.N.A.
- Tower Control - Ability to activate a Tower for X.A.N.A., from the inside or outside. This mimics Aelita's power and it is possible X.A.N.A allowed him to use a version of the Annex Program. Only available under control of X.A.N.A.
In this alternate continuity based off of the pilot of the show, William is mentioned during the second File of the story by Sebastian, who claims to have seen William flirt with girls before. Unlike his Code Lyoko counterpart, the Garage Kids version of William goes to a different boarding school in the area known as Asimov Institution.