Garrett Miller (voiced by Jason Marsden) was first introduced as one of the next generation of Ghostbusters in Extreme Ghostbusters.
Garrett is a lifelong wheelchair user, has a very 'jock'-like attitude and is a huge fan of extreme sports and attempting mad stunts. In the episode "Grease", it is revealed that Garrett was born with the inability to walk and studying to be a physical therapist, and throughout the series he only refers to his condition to mock it (and in "Be Careful What You Wish For", uses it as an excuse to dump some work on Eduardo Rivera). He is the most headstrong and enthusiastic of the new Ghostbusters, often claiming that he is in it only for the adrenaline rush; he was left bitterly disappointed in "Ghost Apocalyptic Future" to learn he was the only Ghostbuster not remembered in the future dystopia. While he studies to be a physical therapist to help other disabled people, his secret dream is to be an NBA star.
In "Fear Itself", it is revealed that Garrett is claustrophobic (though he never admits it to anyone), and has a deep disdain towards the FBI ever since the incident where two agents arrested them under the belief they were causing the sabotages and released a gremlin they recently captured while they were on a plane. He has a rivalry with Eduardo, the two of them constantly bantering and trying to one-up each other. In "Deadliners", we find out Garrett wrote a (bad) horror story starring himself (the other Ghostbusters died on the first page, to their annoyance). Like Ray Stantz, Garrett is the most enthusiastic. Garrett is the only Ghostbuster in the history of the franchise never to be made as a toy (all other Ghostbusters received at least two toys, including Louis Tully and Janine Melnitz), presumably due to the implausibility of having the wheelchair as a separate assembling piece. Though a prototype figure was made.
Bob Higgins has said that during a focus group of young children, the creators found Garrett was the most popular character: "when we asked... which of these characters would you want to be and they all wanted to be Garrett, they all wanted to be the guy that does the crazy things. They all wanted to be the guy that was the leader and they all kind of saw him as the leader of this group [even though he wasn't]".