Emperor Palpatine

Emperor Sheev Palpatine, aka Darth Sidious, is the main antagonist in the Star Wars saga/trilogy, with the exceptions of A New Hope and The Force Awakens.

Star Wars Episode I: The Phantom Menace
Palpatine represents the small, outer-rim planet Naboo in the Galactic Senate. As Darth Sidious, he arrages for the Trade Federation to invade Naboo, knowing that procedural red tape in the Senate will prevent Supreme Cancellor Valorum from taking any action on the invasion. His plan is to use this failure to act to discredit Valorum in the Senate, and elevate himself to the Chancellorship. Valorum sends two Jedi Knights, Qui-Gon Jinn and Obi-Wan Kenobi, to Naboo. They end up rescuing Naboo's leader, Queen Amidala, and escorting her to Naboo. Though this was not part of Palpatine's plan, he quickly takes advantage of the situation, manipulating her into calling for Valorum's removal. As expected, being from Naboo himself gains him a sympathy vote, and he is elected Chancellor.

During the Battle of Naboo, Sidious' apprentice, Darth Maul, kills Qui-Gon Jinn but is himself killed by Obi-Wan Kenobi. Palpatine quickly sets his sights, however, on Kenobi's new apprentice: Anakin Skywalker, whom the Jedi believe to be a prophecied Chosen One who will destroy the Sith.

Star Wars Episode II: Attack of the Clones
Ten years later, Anakin Skywalker has grown into a young adult, and Palpatine has maintained a relationship with him for some time. As Chancellor, he is forced to deal with a separatist movement led by former Jedi Count Dooku, who years before had been Qui-Gon Jinn's mentor.

Secretly, Count Dooku was in fact Sidious's next apprentice, Darth Tyranus. Sidious planned to use this separatist movement to create a war, and use the war as a pretext to gain more executive power and have his own private army of Clones. The outbreak of the Clone War at the climax of this episode gives Palpatine the power he wants.

Star Wars: Revenge of the Sith
Palpatine arranges for himself to be kidnapped by Count Dooku and General Greivous, laying a trap for Anakin Skywalker and Obi-Wan Kenobi, who come to rescue him. Unbeknownst to all but himself, Palpatine intends to use this incident not only to test Anakin's willingness to use the Dark Side, but also to rid himself of Count Dooku. After Anakin disarms Dooku (literally), Palpatine convinces him to kill Dooku. His willingness to do this proves to Palpatine that he is an ideal Sith apprentice.

He spends the next several days grooming Anakin, convincing him that turning to the Dark Side will allow him to save his wife from death in childbirth, and that having a Sith Lord in charge of the galaxy will bring stability. Anakin resists at first, informing the Jedi that Palpatine is a Sith, but later relents, and ends up helping Palpatine kill Mace Windu. In the fight, Palpatine is permanently disfigured. Palpatine accepts Anakin as his apprentice and renames him Darth Vader. He sends Vader to kill all the Separatists, and at the next session of the Senate, declares himself Emperor. Yoda confronts him in the Senate, and the two engage in an epic duel, but it ends in a stalemate, with Yoda going into exhile. Palpatine then travels to Mustafar to rescue Vader, who has been seriously injured in a duel with Obi-Wan Kenobi. To keep his apprentice alive, Palpatine has him encased on a robotic body and suit.

Star Wars: A New Hope
Palpatine is not seen in this film, but we learn that he has disbanded the Imperial Senate, the last remaining aspect of the Republic's government.

Star Wars: The Empire Strikes Back
Palpatine appears only in holographic form in this episode. He warns Vader of the threat posed by Luke Skywalker, but Vader suggests turning him to the Dark Side, to which Palpatine agrees.

Star Wars: Return of the Jedi
Palpatine finally returns in the flesh in this episode, arriving over Endor to oversee the final stages of the construction of the second Death Star, and to witness the destruction of the alliance. As expected, the Rebel Alliance sends a strike force to Endor to deactivate a shield generator that is protecting the Death Star. When Vader tells him that Luke is with them, he instructs him to bring Luke to him so that they can finish converting him to the Dark Side.

Luke is much more resistant to turning than his father was. After being goaded by Palpatine for some time, he finally attempts to kill Palpatine, but Vader comes to his defense, and father and son engage in a lightsaber duel. Luke finally disarms his father, at which point Palpatine urges him to kill his father and become his next apprentice. Unlike his father, Luke refuses, so Palpatine attacks him with Force Lightning, torturing and nearly killing him. Seeing his son nearly dying at his master's hand however reawakens the humanity in Vader, and he hurls Palpatine into the Death Star's main reactor. This kills Palpatine, and fulfills the prophecy that he had spent a lifetime trying to prevent. Unfortunately, Palpatine turns his lightning on Vader, shorting out the circuits in his robotic body and mortally wounding him. Vader accepts his fate, however, and asks Luke to remove his mask so he can see his son naturally. He takes one first and last look at Luke, then dies as Anakin Skywalker.