Fatz Geronimo was the keyboardist, leader, and intended main man of the Rockafire, as shown by his spot in the very center of the show! A lot of fans also see him as a father figure for the band, particularly Mitzi due to their silly, close interactions and age difference. Fatz frequently tells stories and silly anecdotes to the band (but frequently gets interrupted by the band in the middle of them LOL) and introduces shows. He seems to be quite respected in the band, as he is able to shut all of them up and is given the final word on some arguments. Fatz is also shown to be quite the ladies man... he often will flirt with women in the audience even though he's "going steady" with another gorilla named Esmerelda (whom he apparently quarrels with often, according to his stories. I don't blame her, I'd be mad if I saw him "cruising down main street in [his] delta 88 with them six beautiful babes." - Fatz in the Summertime Cyberstar) He had an infamous rivalry with Rolfe, proclaiming that hed "break his neck" after calling him a "big fat monkey," and Rolfe feels similarly towards him, forcing him to play with a broken tune machine because it was "in the contract!" Fatz is also (according to himself) classicly trained in music! After escaping from the zoo, he went to learn to play music at 'Juliart Iglesias School of Music'. After Beach Bear points out that he wasn't actually playing his tune machine during a show, he asks Fatz to play something on the spot and it turns out terrible! That brings us to his tune machine. The tune machine is the name of his keyboard, and it can play every sound in the world! This machine amazes the band, especially Dook. He asked for Fatz to "play every sound in the whole world at the same time but backwards," which failed spectacularly. "Dook, you made me break it!" Outside of this skit from the tune machine cyberstar, the tune machine does in fact play every sound! If the Rockafire wants to play a song with an instrument they don't have, it is played on his tune machine!
I have an absurd amount of fan made lore regarding Fatz that is tied into my fanmade Rockafire comic! I like to imagine that Rockafire Fatz and WP5 Fatz are the same guy, and that he was just very young in the WP5 and older in the Rockafire. Also he is 1000% Mitzi's adopted father because I say so. And he is the dad friend for the rest of the band (except Billy Bob, who I also see as older. They're best friends in my headcanons :3). He also doesn't really hate Rolfe but yeah he gets fed up with him a lot LOL.