The title of emperor (feminine empress) was held by certain officials and heads of state in certain governments. It was usually, but not always, inherited. Sometimes successful warlords and conquerors created empires by declaring themselves emperor.
Emperor Shey Tapani founded the Tapani Empire around 7328 BBY. Empress Teta was a Human female who united the Koros system around 5000 BBY. After her death, the system was renamed for her in honor of her memory.
The title was also used in certain other quarters. Uueg Tching was the fifty-forth Emperor of Atrisia, and Antbbianplourr III was Emperor of Eiattu 6 around 19 BBY. Belgoa was Emperor of Trunska during the Galactic Civil War.
Vitiate was the Sith Emperor following the end of the Great Hyperspace War, and later Emperor of the Eternal Empire of Zakuul as Valkorion. Darth Malgus was the claimant Sith Emperor and ruler of the New Empire movement in 3640 BBY. Arcann was the Emperor of Zakuul and the Eternal Empire between 3636 BBY and 3630 BBY, while Vaylin was Empress of Zakuul and the Eternal Empire in 3630 BBY. Darth Acina was empress of the Sith during the Eternal Empire conquest and the subsequent uprising. Darth Vowrawn was Acina's successor and launched the third Galactic War.
Marasiah Fel, Empress
Assuming power of the galaxy in 19 BBY, Galactic Emperor Palpatine was the leader of the Galactic Empire. He was usually just referred to as "the Emperor." As the Empire's power and influence diminished following the Emperor's death in 4 ABY, however, the title became less important, and by 12 ABY onwards, the Empire was effectively headed by the Supreme Commander in a military dictatorship. The power of the throne was eventually restored under the Fel dynasty: Jagged Fel was the first Emperor of the Fel Empire, taking power around 44 ABY. His successor was Fel II, while the third emperor was Roan Fel, reigning between around 127 ABY to 138 ABY. Darth Krayt became Emperor of his own Sith Empire in 130 ABY and ruled until around 138 ABY; at the time, the Fel Empire was under the command of Empress Marasiah Fel.
The title of emperor was first mentioned in the novelization of A New Hope, ghostwritten by Alan Dean Foster and published in 1976.
The player character in the Star Wars: The Old Republic, known as the Outlander, has the choice of either becoming a diplomat, unifying the Sith Empire and Republic in the Eternal Alliance through diplomacy, or taking the new title Eternal Emperor (the previous title was Immortal Emperor), unifying the galaxy through force.