Clown appeared in the episode entitled "The Thaw" and is part of the archetype known as the "evil clown" - which has become prominent in popular culture ever since the Joker in Batman popularised the subject (though the actual theme has been around at least as long as clowns themselves).
The network had been set up by the Kohl to keep their minds active while they waited in stasis for fifteen years until they could be rescued. Their planet had suffered an environmental catastrophe. Unfortunately they had not foreseen that the feedback between their minds and the network would create a nexus for their darkest fears; essentially, they were so afraid that something would go wrong that the Clown, a malevolent manifestation of their worst fears, was created from their own fear of death, turning their dream state into a violent and torturous environment.
The Clown demonstrated mood swings and erratic behavior. For nineteen years he tortured the inhabitants of the network, killing three of them by subjecting them to the 'illusion' that they were being placed in a guillotine, but at the same time refusing to let them leave as that would end his own existence. The environment was that of a circus, with different characters such as a little woman and a spectre.
In 2372, the crew of the Voyager found the Kohl settlement after receiving an automated signal. Harry Kim and B'Elanna Torres entered into the dream state of the surviving Kohl in order to help them. The Clown threatened to cut off Kim's head but didn't when Viorsa, a Kohl, told him that if he did, Kim's shipmates would turn off the program. Having permitted Torres to leave to explain the situation to the rest of the crew, The Clown subsequently turned Kim into first an old man and then a baby, nearly forcing Kim to relive a memory where he witnessed people having to undergo extensive surgery after an accident. Although Captain Janeway attempted to negotiate with the Clown by sending The Doctor into the environment – as The Doctor was a hologram himself The Clown was unable to tap into his psyche and their attempts to negotiate were unsuccessful, resulting in The Clown killing Viorsa when he found him helping the crew as they attempted to shut down the program. He was tricked into releasing the Kohl and Harry Kim in exchange for Janeway. However, while they connected her mind to the system, they were able to configure it so that she remained conscious, her mind linked to the system without actually becoming part of it, while a hologram was used to fool The Clown into believing she was really there. As the system shut down around him, The Clown admitted his own fear before disappearing into darkness.