On Lyoko, Franz has incredible abilities, but, like XANA, is never seen. He has the power to activate towers to affect Earth (his are white, as opposed to XANA's Red), and to override any XANA-possessed tower. He has also gained the ability to control XANA's monsters.

On Earth, even less is known about Franz Hopper, aside from the fact that he worked on Lyoko. A few facts remain solid: He knew how to play the piano, he was a teacher at Kadic at one time, and had a passageway to the sewers constructed to his own backyard.

His first appearance was in the first episode of the second season, "New Order." It isn't much, but we see him playing the piano in one of Aelita's visions. He has no speaking parts. His name is first mentioned in the next episode, "Uncharted Territory," in which Jeremie reveals that he used to own the Hermitage and also used to be a science teacher at Kadic. In "Mister Puck," Aelita goes back to the Hermitage and finds her doll, in which Franz hid the key to a locker that held his diary. Soon after finding the diary, XANA possesses Jeremie and forces him to destroy the hardcopies, after the data is all saved directly to the restricted section of the supercomputer. In "Marabounta," Jeremie uses data based off of one of Franz's diary entries to create the blobby anti-XANA monster. In "Temptation," Jeremie creates a device based on Franz's designs to increase his already ridiculous IQ with every Return to the Past. In "Franz Hopper," Franz apparently starts helping everyone but Jeremie, but a stupid slip of the tongue gives him away as a Polymorphing Specter (shape-shifting XANA monster.) It is then that Jeremie reveals that Franz is still somewhere in Carthage. In the next episode, "Contact," Franz activates a tower on Lyoko and possesses Sissi, making her write out messages to the group and tries to help them before XANA deactivates the tower using his Megatanks. Once Jeremie deciphers the messages, it reveals that Franz Hopper wants to help them defeat XANA.

In the episodes "Revelation," and "The Key," Franz Hopper plays a major starring role. At the end of "Revelation," Franz takes over the tower that XANA was trying to control and decodes his diary at hyper speed. It is through this that it is revealed that Franz is Aelita's father, that he created Lyoko, and that he and Aelita have the "Keys" to Lyoko. In "The Key," Franz's data reveals that Aelita doesn't have a virus, but is actually missing a piece of herself that XANA stole from her. When they go to retrieve it, it turns out to be a fake, but Franz tries to help with his mind-controlled Mantas. When they are crushed by falling rubble, Aelita is completely drained by the Scypizoa (sp?). When the Scypizoa (sp?) leaves, Franz completely heals her, bringing back all her memories of the real world, which was what XANA took out of her. In the process, he also heals Lyoko, which pretty much shut down once both sets of Keys left with XANA.

It's unknown at the present if Franz is alive or dead, due to his apparent sacrifice to bring back his daughter. However, hints and rumors from the few things we know about the third season seem to indicate that he will play a role in the upcoming episodes.