Dooku

Dooku was a former Jedi turned Sith Lord.

In-Universe Information
Born to a noble family on the world of Serenno, he was found to be Force-sensitive and taken to be trained as a Jedi. Later in life, angry over the death of his former padawan, Qui-Gon Jinn, he left the order, becoming only the twentieth Jedi in history to voluntarily renounce his commission. He joined with Darth Sidious and became a Sith Lord called Darth Tyranus.

Portrayals in Star Wars Media
Dooku was played by legendary actor Christopher Lee in episodes I and II and voiced by Corey Burton in Star Wars: The Clone Wars.