Yan Dooku

Count Yan Dooku was a human male Jedi Master who served the Galactic Republic in the decades prior to the Clone Wars. Born into a noble family on the planet Serenno, he was the heir to vast wealth and the noble title of Count. Dooku was taken by the Jedi Order as a child and apprenticed to Thame Cerulian. As a Jedi Knight, he took Qui-Gon Jinn as his first Padawan, and later trained Komari Vosa. Dooku was a respected instructor in the Jedi Temple and one of the most renowned swordsmen in the galaxy, surpassed only by Grandmaster Yoda and equal to Master Mace Windu.
Dooku spent nearly seventy years as a Jedi, but a disastrous battle on Galidraan shook his faith in the Order and the Galactic Republic. He fell under the influence of Supreme Chancellor Sheev Palpatine, formerly Senator of Naboo, and left the Jedi Order. After learning that Palpatine was secretly the Sith Lord Darth Sidious, Dooku turned to the dark side and became a Dark Lord of the Sith himself, replacing Darth Maul as Sidious's second apprentice. His new master bestowed upon him the Sith title of Darth Tyranus.