He appeared in the episode Claustrophobia. He showed to having an interest in Sissi in this episode, resulting in jealousy from Herb Pichon. Although in a Time Reversion, Odd instead sparks an interest in him towards Yumi in order to prevent rivalry between Theo and Herb. However, this also is shown to have made Ulrich jealous when Theo offers to walk Yumi home from a soccer game with his arm around her.
Theo has made non-speaking appearances in other episodes. He was a "background character" for those episodes. Theo has made these "appearances" in at least 1 episode in Season 2, Season 3 and Season 4.In Code Lyoko Evolution, X.A.N.A. created a spectre of a boy with a very similar appearance with Theo in the episode How to Fool X.A.N.A. to hypnotize Yumi into giving him her codes. However, it is very unlikely if the spectre was really Theo, despite their similarity.
|Jeremie Belpois | Aelita Schaeffer | Odd Della Robbia | Yumi Ishiyama | Ulrich Stern | William Dunbar|
|Sissi | Jim Morales | Suzanne Hertz | Anthea Schaeffer | Mr. Delmas | Laura Gauthier | Franz Hopper|
|Kiwi | Herb Pichon | Nicholas Poliakoff | Amelia "Milly" Solovieff | Tamiya Diop | Hiroki Ishiyama | Johnny | Takeo and Akiko Ishiyama|
|X.A.N.A. | Monsters|
|Professor Tyron | Ninjas | X.A.N.A. William|
|Jean-Baptiste Pujol | Theo Gauthier | Emily LeDuc | Taelia | Samantha Knight | Samantha Suarez | Heidi Klinger | Mathias Burrel | Magali de Vasseur | Claire Girard | Brynja Heringsdötir | Patrick Belpois|
|X.A.N.A. Aelita | X.A.N.A. Odd | X.A.N.A. Yumi | X.A.N.A. Ulrich | X.A.N.A. Franz Hopper | X.A.N.A. Anthea Schaeffer | Jeremie Clone | Aelita Clone | William Clone | Subdigitals|