Vex, was an assassin. She would kill for any price. In her past days, she killed three Jedi, one of them included a Padawan.
However, one day, she gets hired by the Separatists, and is given a lightsaber, trained by Count Dooku. She isn't the strongest Force user, but she can still use it however.
She gets taught in the way of the Dark Side, and eventually, she gets sent to war, to kill a certain Jedi: Anakin Skywalker. She finds her way around to him, but there is a... distraction, and somehow, winds up into the arms of Anakin Skywalker, aboard his ship.