Marvin is one of Simon's sidekicks.
He first appeared in Secret of the Hidden Cave Part l where he and his boss, Simon, and Vinnie Vincent, were trying to buy the Brenan Ranch. After the Brenans refused to sell the land, he, Simon, and Vinnie set fire to the Brenan's barn. The threesome are finally caught in Secret of the Hidden Cave Part ll when they try to steal a fossil from the Hidden Cave.
Later, in Return to Hidden Cave, he, Simon, and Vinnie try to escape from jail, through a tunnel dug with just mere soup spoons. They begin their escape but are caught before they make it out of the guard fence. They are then transferred to a high security state penitentiary after said escape. After arriving at the state penitentiary they meet a woman named Miranda. Miranda promises to get them out of jail just as long as they help in an attempt to steal highly classified computer chips. With no intention of actually helping Miranda, Simon agrees to the bargain. Once they escape from jail however, Simon, Marvin, and Vinnie are forced to help Miranda or she threatens to turn them in. Miranda then tells them to hid the computer chips in the Hidden Cave. Simon reluctantly agrees to the deal and they stow the computer chips in the cave, but after Miranda realizes that the Cave is supposed to be a large attraction at the Brenan Museum of Creation, she orders Simon, Marvin, and Vinnie to go guard the computer chips until they can move them. While guarding the chips the threesome are captured and returned to jail.
He appears again in Descent from Sandia Peak, where he, Simon, and Vinnie are working at Sandia Peak picking up garbage as part of their sentence. At Sandia Peak he, Simon, and Vinnie find themselves stuck in a tram with Mrs. Park, Ryan, and Mrs. Brenan. While on the tram Mrs. Park begins to have contractions. Simon, Marvin, and Vinnie begin to worry that Angela is going to have her baby on the tram, but Jonathan and Jessie fix the tram and Simon, Marvin, and Vinnie are forced to take Mrs. Park, Mrs. Brenan, Ryan, Jessie, and Jonathan to the hospital. After this excursion they are released and are not heard from again for a while.