Asajj Ventress once was trained under a Jedi named Ky Narec prior to the events from the Clone Wars. When her master was killed by space pirates, she took revenge on them and killed them out of anger. In doing so, she crossed path into the Dark Side of the Force. Soon afterwards, Count Dooku saw her potential and trained her as his apprentice, thus making her merciless as she proceeded to battle against the Jedi.
Overtime however, her power with the force apparently got on the nerve of Darth Sidious (Emperor Palpatine). Fearing that she might overpower him eventually as the new Sith Lord, Sidious ordered Count Dooku to kill her. After narrowly escaping death, Ventress returned to her home planet, where she reconciled with the Nightsisters, there she began plotting vengeance against Dooku. Before the assassination plot was put into motion however, General Grievous intercepted the Nightsisters and killed them all, except for Ventress.
Ventress then retreated to Mos Eisley, where she met Boba Fett and briefly worked with him before parting ways. Her complex character showed a noble quality to her, such as later on as The Clone Wars progressed, she would also fight alongside Obi-Wan against Darth Maul, as well as assist Ahsoka Tano after she was falsely accused of crimes by the Jedi Court.
Eventually, when the Jedi wanted to learn more about Count Dooku and his whereabouts, they sent a Jedi named Quinlan Vos to find her and gain her trust. Ventress learn about the his scheme as a spy but nonetheless agreed to assist him in some form, largely because the two had similar personalities. Later, she also was smitten by him. In her last moments, she sacrificed her life for Vos, never succeeding in her go to kill Count Dooku and get her revenge on him.