In the film, Fungus is Randall's scare assistant. He's rather cowardly and wimpy, due to the wrathful temper of Randall, and helps him with his plot to revolutionize the scaring industry, although he regrets it.
He helps Randall (out of fear) to create the Scream Extractor. Fungus is initially a kind person and means well, even apologizing to Mike about his capture. Due to Sulley's timely intervention, Fungus falls victim to the machine himself, which blanches him white, proving that it would cause great physical harm if used on a human child (or, in the worst case scenario, certain death).
Fungus is later seen operating machine's controls once again, this time testing it on Boo as Randall and Waternoose watch on in anticipation. He also assists Randall as they pursue Mike and Sulley (who had rescued the girl from the device) but falls behind due a crowd of monsters congratulating Randall for supposedly breaking Sulley's Scare record, courtesy of Celia at the PA system's controls.
After Randall's banished to the human world, Fungus is a lot happier and takes on the job of making children laugh. He's also grateful to his new boss, Sulley, both for forgiving him for conspiring with Randall earlier and assigning him a job on the newly-commissioned "Laugh Floor." It's unknown if he also became a scarer, as it was revealed in Monsters, Inc. Scream Team Training that the monsters still use screams.
Fungus doesn't appear in Monsters University, which means that he probably met Randall after they started working at Monsters, Inc. Though, one of the ads in the credits was labeled "Fungus's Friends."
Fungus is shown to be very clumsy and cowardly, the latter proven when Randall threatens him to put him through the shredder if he doesn't bring another door to the station. He is generally incompetent and always bullied by Randall.
Fungus is initially a kind person and means well, even apologizing to Mike about his capture. He eventually falls victim to the machine himself. After Randall's banished to the human world, Fungus is a lot happier and takes on the job of making children laugh.
- Fungus isn't really evil, as he only serves Randall because he fears him.