The Trickster (The Sarah Jane Adventures)

The Trickster is a being from beyond the universe that seeks to manifest itself via chaos and has become a recurring antagonist in the Doctor Who universe - most notably the portion dealing with Sarah Jane (though the Trickster was mentioned in the official series during the events of "Turn Left").

The Trickster can not exist for long periods of time in this universe and can't take physical form, often having to employ reflective surfaces in order to "show" itself - although seemingly working alone the Trickster is said to be part of a Pantheon Of Discord.