A former Jedi Knight who left the Order when he became disillusioned with the corruption in the Republic, Dooku is the public face of the Separatist Alliance leadership. Many see him as a charismatic politician, but he has in truth been corrupted to the dark side by Darth Sidious.
Sidious corrupted Dooku, luring him to the dark side by appealing to his disgust with corruption inherent in the Republic. Dooku brought to the Sith decades of skill and knowledge as a Jedi, and adopted the title Darth Tyranus.
Asajj Ventress loyally served Count Dooku. She aspired to be a Sith Lord, and his secret apprentice. However, when Darth Sidious grew wary of Asajj's abilities and feared that Dooku might be plotting a move against him, Sidious ordered Asajj eliminated. It was an act of cold betrayal that Dooku turned his back on Asajj.
After Dooku was forced by Darth Sidious to dispose of Asajj Ventress, he turned to the Nightsisters to deliver a new apprentice. Dooku was a cruel master to Savage, attempting to hone his apprentice's rage into a powerful weapon.
A charismatic firebrand who spoke out against the corruption of the Republic, Count Dooku was able to rally thousands of planets into declaring independence from the galactic government. Dooku felt the Jedi dishonored themselves by serving the Republic, and thus could not be trusted.
Some believe Qui-Gon Jinn inherited his maverick ways from his former Jedi Master, Dooku. The death of Qui-Gon preceded Dooku's departure from the Jedi Order.