In Godzilla vs. Megalon, Jet Jaguar is a robot created by the protagonist, Goro Ibuki. He is initially depicted as a mindless robot, and is controlled by the evil Seatopians for most of the film. He later gains sentience and helps Godzilla battle the evil Megalon and Gigan. He is depicted as a heroic character with the abilities to communicate with kaiju, fly, alter his own size, and use floodlights in his eyes.
He was even given a brief theme song at the end of the film, performed by Masato Shimon.
Jet Jaguar was the result of a contest Toho had in mid-to-late 1972 for fans to come up with a new hero for them to use (to capitalize on the many tokusatsu and anime superhero and super robot shows that were all the rage at the time). The winner of the contest submitted the drawing of a robot called Red Arone. The robot resembled both Ultraman and Mazinger Z (both of which were very popular at the time). The robot was renamed Jet Jaguar and was set to star in a film vehicle for him, titled Jet Jaguar vs. Megalon, which pitted him against Megalon (a previously unused Godzilla villain). However, Toho figured Jet Jaguar would not be able to carry the film on his own, in screen appearance or marketing value, so during pre-production, even after doing some tests and storyboards, the project was shut down for several weeks, until screenwriter Shinichi Sekizawa was called in to rewrite the script to add Godzilla and Gigan to have more marquee value. The resulting film, Godzilla vs. Megalon, paired Godzilla with Jet Jaguar against both Megalon and Gigan.