The Trickster

The Trickster was a strange being that thrived on chaos. It could also remove beings from, and so influence the course of, history. It had a mouth of sharp teeth, with no eye sockets and was always hooded. It could materialise and dematerialise at will, appear to certain individuals and appear either in reality or only in a mirror.

He could only influence reality with the consent of another, and otherwise was powerless. (SJA: Whatever Happened to Sarah Jane?)

History
Nothing is known of the Trickster's origins. All that is recorded is his attempts to influence history.

To satisfy its need for chaos, it found one main event in 2007, which if it happened would have caused destruction on Earth and wiped out Humanity, who would later become a major species in the Universe. This event was that of a meteorite crashing into the Earth, undetected by military radar. There were many other times when life on Earth would have been threatened, however the meteorite crash was one of those few which had no purpose, it just being the reult of pure chance.

In the original timeline, the impact would have been stopped by Sarah Jane Smith and Mr. Smith. To stop this, the Trickster removed Sarah Jane Smith from Earth's timeline, influencing events so she died in the place of Andrea Yates (who should have died in the original timeline) in 1964.

The Trickster also stopped the aliens that Sarah Jane would have faced to leave the Earth alone, so Luke Smith was never born because the Bane never invaded.

However, all did not go to plan. When removing Sarah Jane from history, one night Maria Jackson held an alien cube, protecting her from the effects and allowing her to remember the original timeline. She reminded Andrea Yates of the say Sarah Jane died, causing Andrea trauma. The Trickster hired a Graske to remove Maria and later removed her from history, however her father (Alan Jackson) found and held the cube at the point of Andrea's consent, allowing him to remember the events. In turn, he reminded Andrea of Maria, again causing her distress. The Trickster sent the Graske after Alan, however Alan foiled the plan and trapped the Graske.

He discovered a device on the Graske which summoned Maria back (from Limbo), and the two left the Graske and went to Sarah Jane's house (in this timeline, Andrea's) where Sarah Jane materialised in a mirror. She explained that while Andrea was alive, she couldn't be. Andrea, realising that she had to die (either by falling of a pier in 1964 or in the soon-to-be meteorite crash), went back on her deal with the Trickster. The timeline was back to normal, Sarah Jane and Mr. Smith deflected the incoming meteorite and the Trickster was defeated.

Had his plan suceeded, the Trickster said that he alse wished to use Sarah Jane to locate the Doctor and remove him from time, creating a lot of chaos throughout the Universe. (SJA: Whatever Happened to Sarah Jane?)

Behind the Scenes
The name of "the Trickster" was only given to the being by Alan Jackson as a description of him, and as such the being was refered to as "The Trickster" in the end credits. Its actual name, if it had one, is unknown.