The Clown was a manifestation of the fears of the individuals who were part of a linked neural network.
The network had been set up by the Kohl to keep their minds active while they waited in stasis for 15 years. Unfortunately they had not foreseen that the feedback between their minds and the network would create a nexus for their darkest fears.
The Clown was clearly insane demonstrating mood swings and erratic behaviour. For 19 years he tortured the inhabitants of the network, killing three of them before he was defeated by the crew of the USS Voyager. They tricked him into releasing the Kohn in exchange for the mind of Captain Janeway. However, while they connected her mind to the system she stayed conscious and 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. (VOY: "The Thaw")