Grand Moff Tarkin

"You would prefer another target, a military target? Then name the system! I grow tired of asking this so it will be the last time: Where is the rebel base?"

- Grand Moff Tarkin's interrogation with Princess Leia.

Grand Moff Tarkin is the secondary antagonist  of Star Wars Episode IV: A New Hope. He is an Imperial commander for the Death Star. He acts as Darth Vader's collegue for giving commands and creating plans to destroy the Rebellion. He was portrayed by Peter Cushing. He also makes a brief cameo in the last movie of the saga Revenge of the Sith at the end of the movie as he along with Vader and Emperor Darth Sidious supervise the construction of the Death Star on board the Star Destroyer.

Episode IV: A New Hope
In the first officially launched episode of Star Wars, Grand Moff Tarkin (Peter Cushing) makes a first appearance with Darth Vader, after the Millenium Falcon had landed one one of the Death Star bays. Darth Vader feels the presence of his old master, but Tarkin reassures him that the Jedi are extinct.

He and Lord Vader held Princess Leia Organa captive on board the Death Star. They attempted to force her to reveal the location of the Death Star. After Leia told him the base was on the planet of Dantooine, Tarkin decides to destroy the princess's home planet Alderaan anyway, and then deal with her "rebel friends." After the truth is revealed that the rebel base was not at Dantooine, Tarkin planned to have Leia Organa killed.

After Princess Leia is rescued by Luke Skywalker and Han Solo, Tarkin plots with Vader the destruction of the Rebel base when in range. During the Rebel attack on the Death Star, however, Luke Skywalker using the Force to guide his proton torpedos to the exhaust port destroys the Imperial space station, killing all Imperial troops, commanders, and officials on board, including Tarkin.

Trivia

 * Tarkin is regarded as the main antagonist of the original "Star Wars" movie, as Emperor Palpatine/Darth Sidious (the true villain of the entire saga) did not make his first appearance until "The Empire Strikes Back", and also Peter Cushing, who played the character got high billing behind Mark Hamill (Luke), Harrison Ford (Han Solo), and Carrie Fisher (Princess Leia).
 * Despite the fact that Luke, Han Solo, Chewbacca, C-3PO, R2-D2 and Obi-Wan Kenobi are Tarkin's enemies, they have never had any actual interactions.