Felix Gumm is an OSS agent of a lot of importance to the Cortez family, from being with them since the Cortez children's birth.
In Spy Kids, 'Uncle' Felix pretended to be Carmen and Juni Cortez's uncle, meaning that he was Gregorio Cortez's fake brother, in place of the cut off Machete Cortez. He wears a fake mustache to fool the Cortez Children, which convinced them very much throughout the very first Spy Kids film. Felix was later caught by Ninja Thumbs and turned into a Fooglie just like the other captured agents. He was presumably switched back after the film.
Spy Kids 2
In Spy Kids 2, he vaguely showed up at the beginning of the film (as he was seeing off Carmen and Juni Cortez before their mission), and then appeared rather widely at the end, when he was forced to attempt to betray the Cortez family, including Helga and Valentin, as he was persuaded by Donnagon Giggles, the so-called new Director of the OSS, and a traitor who tried to take over the planet with the Transmooker.
Spy Kids 3
He briefly showed up again, this time to help the Cortezes fight the Toymaker's virtual robots. This time he called himself Carmen and Juni's uncle despite denying it in the past two films.
- Even though he's not their uncle, he later finally accepted it at the end of the third film.