Diesel is fun-loving and is a silent and often unintelligible character. When given a voice box, Diesel's personality seems to alter, which is first shown when Lugnut gives him one to make him sound like what seems to be a over-excited sports' announcer and causing him to become crazy such as whacking Lugnut in the chest and running in circles until he is tripped by Lugnut. He then acts like a dog, growling at Crank when he tries to remove the voice box and even gaining "puppy-dog eyes" and sticking out his tongue.
Diesel is a small, dark blue robot with bright green eyes, which seem to have glass over them as shown when they randomly shatter when Madame Gasket says she's a woman. He has a small body with long arms and legs, and a very large, flat head. He has what appears to be some sort of dial for a nose, and two switches atop his head that he uses as an instrument towards the end of the movie before singing. He appears to have a light pink tongue. During the final fight, he is "upgraded" to look like a cowboy.
Diesel is missing a voice box, meaning he cannot speak. However, whenever he puts a new voice box in, he gains a matching personality. For example, after Lugnut unknowingly gives him a dog's voice box, he begins to bark like a dog and growls at Crank. Diesel lives with Aunt Fanny, Fender, Piper, Crank and Lugnut at the boarding house. At the victory celebration in Rivet Town, Bigweld gives him a new voice box, which causes him to gain the singing voice of James Brown. After using the two switches on his head as an instrument, he proceeds to sing "Get Up Offa That Thing" as the rest of the characters dance along.
Diesel first appears with Fender at the train station, trying to scam Rodney out of his money. He seems to function as a cash register at that point. In the next scene where Fender and Rodney are riding to Bigweld Industries, Diesel does not appear with Fender.
He appears a few minutes later, with Crank and Lugnut looking for parts when he and Lugnut try two voice boxes for him, revealing his name and that Diesel is mute. Just as Diesel gets a dog voicebox, Fender enters the area. When Fender tells the others that Rodney intentionally knocked his head off, Diesel and Crank surround him possibly planning to attack him, but him and Crank are presumably stopped by Piper.
In Jackhammer's Hardware, he appears in the background with the other Rusties. He backs up Piper when she scolds Jackhammer for not having parts for Fender and takes on a Mufasa voicebox, a Sally voicebox, and a Dr. Finklestein voicebox when Rodney tells Fender he can fix him.
He appears with the other Rusties going to bid Rodney farewell. But they are stopped by Bigweld. When they stop at Bigweld Industries for Bigweld to fire Ratchet, he tells Lugnut, Crank, Diesel, Piper, Fender, and Aunt Fanny to stay and "watch Daddy's limo." But later, Diesel is shown partying in the front seat of the limo with Crank, Fender, and Lugnut poking themselves out from the top of the limo, chanting "Road Trip!", to Piper's annoyance. When he notices Rodney in trouble, Diesel is the one driving the limo.
He upgrades to resemble a cowboy and wields two gun-like weapons. He also helps Lugnut at one point in the battle.
At the end, he, along with Piper, Lugnut, Crank, and Wonderbot, he assists turning Herb's trumpet playing into a street rave. Bigweld hands Diesel a box containing a James Brown voicebox for himself. He then grabs the microphone and begins to sing "Get Up Offa That Thing".
- He is the smallest of the Rusties, Piper being slightly taller than him.
- His last name was never confirmed anywhere else but merchandise.
- Whether Diesel had an original voice or not is currently unknown.
- He appears to be able to do high jumps and handsprings, indicating he could be athletic.
- He is like a record player.
- He is like Marvin the Paranoid Android.