Jean-Pierre Delmas, known by the students at Kadic as Mr. Delmas and to Sissi as Daddy or Dad, is the headmaster/principal of Kadic. He is very straightforward and weak-minded, as his daughter Sissi can often boss him around, sometimes to the advantage or disadvantage of Team Lyoko. He is very ignorant and gullible as seen in his dealings with his daughter and members of Team Lyoko. She has convinced him many times to organize school dances and other activities for her (or rather, has demanded for him to do so). He often does the announcements at school via the PA system, as most principals of schools do. He often evacuates the school, should there be a problem, like a XANA attack. It's unknown what became of his wife, though Sophie Decroisette implies divorce in an interview. Notably, when communicating with Kadic's student body, he uses the term "children" to address them; ex. "Children, have any of you seen William?", "Children, please give me your attention." (Nicole Weber did this as well, in Unchartered Territory).
He sometimes can be overly authoritative, as seen in Satellite when he quashes the "Bring Back Mobiles!" protest and is yelling at his own daughter. However, he also offers compassion, sympathy, and support as seen when Ulrich asks to go to the hospital with Odd in Killer Music - at first he tells him that he can't let him leave, but realizing that Odd is indeed about to die he is forced to let Ulrich accompany Odd to the hospital.
He is voiced by Alan Wenger, who is also the casting director of the show.
- He fired Jim in Code: Earth, because he felt that Jim had hurt Jeremie, but a time reversion in False Start made him forget all about the incident and allowed Jim to have his job back.
- He was possessed by XANA in Ultimatum and again in Wrong Exposure.
- He has been in the factory multiple times to reprove/scold Jeremie before returning to the past and losing his memory of the factory.
- It's hinted by his own words in Echoes that he is quite generous when it comes to Sissi's allowance, but he refuses to give her another cent. Several other episodes reveal that he could also be strict concerning his daughter's academic performance, as he grounds her for failing the science final in Final Mix. Beyond this, he rarely ever punishes her in the show.
- His personality has, like Jim's, also changed for the worse. In Season 1, he was usually lenient and calm except when the student body revolts against his anti-cell phone policy. In Season 4, he is much more stern, cold at times (especially around his daughter Sissi) and stricter. However, his personality remains congenial and convivial in the video games to appeal to younger fans.
- William Returns and Straight to Heart reveal that Jean-Pierre has several pictures of his daughter on his office desk. Next to it is a frame for another picture of Sissi.
- In Wrong Exposure, it is revealed he knew Franz Hopper, and so did Sissi, years ago before Aelita was born and that Franz was a teacher at the Academy.