He is a small red monster with three eyes and two legs similar to those of a chicken. He wears three-lensed glasses.
In the film, Fungus is Randall's scare assistant. He is rather 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. He eventually falls victim to the machine himself, which blanches him white, implying that it would cause great physical harm if used on a child. After Randall is banished to the human world, Fungus is a lot happier and takes on the job of making children laugh. He is also grateful to his new boss, James Sullivan, both for forgiving him for conspiring with Randall earlier and assigning him a job on the newly commissioned "laugh floor".
Fungus is due to make an appearance in the prequel to the original film. It can be assumed that it will explain how he comes in league with Randall.