Darth Sidious

Warning: Wikipedia contains spoilers.

A Sith lord, Darth Sidious was the prime practitioner of "The Dark Side of the Force". In The Phantom Menace, Sidious sent out his apprentice Darth Maul to kill Queen Padme Amidala and the Jedi knights Qui-Gon Jinn and Obi-Wan Kenobi.

In Attack of the Clones, Sidious again plotted against the Jedis, this time sending his new apprentice Count Dooku, a.k.a Darth Tyranus to do his work.

Later, supposedly in Star Wars Episode 3, Sidious is revealed to be actually Supreme Chancellor Palpatine, and takes on Anakin Skywalker as his new apprentice under the name Darth Vader. It is presumed that between Episode III and Episode IV, Darth Sidious becomes the Emperor who is seen in Episodes IV, V, and VI.

There are clear clues to this fact earlier, for example in the closing scenes of The Phantom Menace, Palpatine is shown in profile, matching Sidious seen in the same position but with his face hidden by the hood of his cloak. Both Darth Sidious and Palpatine are also played by the same actor, Ian McDiarmid[?].