Timothy, later known as Mutagen Man, is a minor antagonist in the 2012 Teenage Mutant Ninja Turtles series. He is a teenager that idolized the Turtles and wanted to become mutant vigilantes like them, only to end up turning on them upon becoming a horrifically deformed monster.
In "The Pulverizer", the Turtles encounter Timothy as he is attempting to stop the Purple Dragons, although, due to his lack of fighting skills, the Turtles are forced to step in and save him. Upon seeing his idols, Timothy explains that he had witnessed the fight between the Turtles and Baxter Stockman and had been inspired to become a hero like them, although not realizing that he had little to no capability of doing so. The Turtles attempt to defeat the Purple Dragons, however, because of Pulverizer's attempts to "help", the Dragons end up getting away with the items they stole. Timothy then gets his bandanna stuck in the Shellraiser as it drives away, resulting in the Turtles unknowingly bringing Timothy back to their lair, although he was luckily rendered unconscious and had no idea how he got there.
With Pulverizer in the lair, Donatello attempts to teach him self defense in order to give him some combat experience, but ends up failing at doing so. After Raphael is bitten and poisoned by Fishface, Donatello has Timothy drive to the warehouse he is in and gets him to jump a roadblock. After injecting Raphael with the antidote for Fishface's venom, Donatello realizes that the jump broke the lead casing covering the Kraang Crystal powering the Shellraiser, and before they can get the Shellraiser and the Crystal back to the lair, the Kraang attack the warehouse and steal the Crystal. Against Donatello's warnings, Timothy runs at the Kraang carrying the Crystal and performs the leg sweep technique he learned, although it fails to work against the Kraang, resulting in them getting away. Timothy initially blames himself for the Kraang escaping, although Donatello acknowledges that he was mostly responsible for the Kraang getting the Crystal. Donatello then tells Timothy that he won't be learning with him anymore, although Timothy vows to continue training.
In "The Pulverizer Returns!", Timothy joins the Foot Clan after enrolling in one of Chris Bradford's dojos. The Turtles spot Timothy attempting to break into a building and confront him, where he explains how he joined the Foot and his position as Oshegotusha, which Donatello explains is really cannon fodder. Realizing that the Foot Clan is evil, Timothy decides to stay on the Foot as an undercover agent, which the Turtles, needing the information, don't entirely disagree with.
Pulverizer later texts the Turtles and informs them of the Foot's next move, which is stealing several Mutagen canisters from a warehouse. After stealing the Mutagen, Shredder orders Dogpound to take a convoy of Ninjas to a warehouse to test the Mutagen on them, starting with Timothy. An exited Timothy then texts the Turtles and tells them that he is going to become a Mutant like them, which immediately horrifies them.
The Turtles locate Timothy and attempt to stop the experiment, although interference from Dogpound, Fishface and the rest of the Foot prevents them from doing so, and Timothy is mutated. However, due to a lack of recent contact with any animals or plants, the Mutagen backfires and turns Timothy into a monstrous blob of organs. Enraged at his transformation, Timothy begins attacking everyone and is left behind as the Foot rig the entire building to explode. The Turtles (specifically Donatello), refusing to let Timothy be destroyed, trap him in the empty Mutagen container and get him out before the building explode. Donatello them brings the blob-like Timothy to his lab and vows to cure his condition.