Asajj Ventress grew up on Rattataki. When the man who raised her was killed in war, a Jedi Master named Ky Narec found and began training her. The two went on to become heroes, fighting unknown to the Jedi Council for peace. They became heroes. But their enemies struck back, killing Narec and orphaning Ventress once again.
“On her own, Asajj would have died. Left to his own devices, the stranger would have undoubtedly been captured and killed. But together…they became something our world had never known…they became heroes.”
―Osika Kirske, one of those who fought against her and Narec.
Ventress wielded 2 lightsabers, hers and her former Master Narec’s, until Dooku gave her new ones, with curved hilts and red blades.
She blamed the Jedi for abandoning her Master on her world and never aiding him or their people. The Jedi, however, had presumed him dead.
Filled with anger, she sought out Count Dooku to train her as a Sith. She won the right to serve him in a gladiator tournament. Her first assignment was to aid in a biologic weapon assignment on Oma Dun, moon of Naboo. Here she battled Obi-Wan Kenobi and Anakin at the same time and proved a formidable foe. She was ordered directly by both Dooku and Sidious to kill the padawan Anakin Skywalker. They clashed over Muun before leaping to He fought her in the sky and on planet before defeating her for that day on Yavin 4.
“Come, Padawan, your fall will be my ascension to the Sith!” ~ Ventress to Anakin
Ventress eventually resurfaced, continually clashing with Obi-Wan in particular. After she captured him and he escaped, she bore a grudge. She was particularly upset he’d gotten away with her Jedi Master Narec’s lightsaber.
She also, critically, discovered Anakin’s relationship with Padme and threatened to kill her. Anakin again defeated her, apparently to death. But unconvinced Obi-Wan sought her out, again. And again, he found her.
At one point she attempted to assassinate Master Yoda, who was on a secret mission.
“Confusion to the Enemy” ~ Ventress, considering the confusion of the Republic forces on Yoda’s death….even though she knew that it was a Yoda impersonator she’d taken and they didn’t.
Ultimately Dooku would betray Ventress and she faked her death to escape.
The Clone Wars TV series throws in some additions:
It suggests she was taken to Rattak as a child by someone who took her from Dathomir, making her akin to the witches.
It also suggest that for a time at some point after breaking with Dooku, she worked as a bounty hunter and even at one point teamed up with Ahsoka Tano. However, due to the cancellation of The Clone Wars TV show and discontinuation of the Expaned Universe, this ends any contributions to her fate into the Expanded Universe Canon.
Dark Horse Comics:
Obsession (mini series)
Star Wars Clone Wars by Genndy Tartakovsky, an animated microseries. There is also a flash game based on it, duel as Anakin against her here.
The Clone Wars
The Cestus Deception
Yoda: Dark Rendezvous
Jedi Trial (Briefly)