My analysis on Vestara Khai will be drawn from the following sources: The Fate of the Jedi series from Del Rey, Star Wars Insider 125, Imprint, and Star Wars: The Essential Reader's Companion.
Who is Vestara Khai?
Vestara Khai is a Force-sensitive female Human who was born on the remote planet Kesh in 27.5 ABY to Sith Saber Gavar Khai and his wife, Lahka. She trained as a Tyro in the planet's local Sith Tribe and later became an apprentice to Lady Olaris Rhea after being contacted by the Sith Meditation Sphere Ship in 41 ABY. Vestara is known for her cunning and skill in lightsaber combat, and has been involved in several key events in the Star Wars Expanded Universe, including the Second Galactic Civil War and the Lost Tribe of Sith's attempts to return to power.
Vestara Khai Skills
Vestara was skilled with a lightsaber and was completely ambidextrous. As a Tyro, she often sparred with her friend Ahri Raas, frequently besting him in their matches. During the mission to Sinkhole Station, Vestara and Rhea briefly dueled Luke Skywalker, and though Rhea was killed, Vestara survived the conflict which says quite a bit about her skill as Skywalker was the strongest Force User and best lightsaber duelist the Jedi Order had.
Vestara Khai & Ship (Sith Meditation Sphere)
Ship helped the Tribe travel off world and form a new fleet, the Sith Armada. However, in 43 ABY Ship deserted Kesh, following the call of the dark side entity known as Abeloth.
Vestara was a member of the strike team sent to recover Ship and kill Jedi Grand Master Luke Skywalker, but the Sith were defeated by Skywalker and his son, Ben. The team's sole survivor, Vestara fled to the planet Dathomir, where she sent a message to her Tribe and lived under the protection of the Dathomiri Witches. The Skywalkers found and eventually captured her, but they were then confronted by a Sith fleet led by Sarasu Taalon. Taalon forced the Jedi to accept an alliance to confront Abeloth, who had proven to be a danger to the Sith, and who was causing a psychosis among the Jedi Order's Knights, (a psychosis where the Knights thought that ones they thought they knew had actually been replaced by body doubles) with the stipulation that Vestara remained aboard the Skywalkers' family ship, the Jade Shadow.
During the mission, Vestara and Ben began to develop romantic feelings for each other, and she was instructed by her father to use them to her advantage against the Jedi. They eventually arrived on Abeloth's planet in the Maw. This same planet was controlled by the Ones. The same group that Anakin Skywalker, Obi-Wan Kenobi, and Ashoka Tano encountered during the Clone Wars. While there, the Sith betrayed the Jedi, but Skywalker still defeated Abeloth. The Khais, the Skywalkers, and Taalon repaired their alliance and remained behind to investigate more into Abeloth, but after discovering that she had survived, the alliance again fell apart. Gavar and Taalon sent Vestara to travel with and spy on the Jedi, and they tracked Abeloth to the moon Pydyr. A contingent of Sith soon arrived, and upon finding Abeloth, Taalon, who had begun to turn into the same kind of entity as she, after a bath in the Pool of Knowledge, entered battle with the Jedi.
The Sith and Abeloth incapacitated the Jedi, but Vestara killed Taalon to prevent his transformation. Abeloth then escaped, and Vestara fled with the Jedi, as the Sith suspected her of betrayal. She and the Skywalkers then located Abeloth on the world Nam Chorios, and Vestara secretly informed her father of their location. However, Abeloth survived the assault and Vestara again assisted the Jedi in escaping from her Tribe.
As the hunt for Abeloth continued, Vestara, the Skywalkers, and Jedi Knight Jaina Solo, who had recently joined them on their journey to Korriban, dueled a group of Sith including Vestara’s father. She ignited her lightsaber and stood against the Sith with the Jedi. He told her that she had brought shame upon their family and accused her of betraying the Sith. Vestara, however, noticed that Gavar, who had taught her that she must always be in control of her actions and emotions, was fighting and acting out-of-control; and she realized that he had not stayed loyal to the Sith, either. Vestara was forced to kill her father. The Skywalker/Solo family comforted her, and Jaina told Vestara that she had to kill her own brother, Darth Caedus, to save the lives of many. It was the greater good.
She finally decided to turn her back on the dark side and train as a Jedi under the tutelage of Luke and she and Ben began a relationship. They soon embarked on a mission to hunt down Abeloth and Ship, but Vestara killed Jedi Knight Natua Wan to save Ben from a deadly insect that was determined to feed on him. The Jedi were forced to flee the world, and due to her actions, Vestara concluded that she would never be able to follow the Jedi path.
Vestara Khai & Ben Skywalker
Vestara nevertheless remained with the Jedi for their mission to eliminate the Sith on the planet Coruscant. She hoped to ensure the destruction of the Tribe's High Lords, who were trying to kill her for Taalon's murder. However, during the mission she was separated from the Jedi, and she was forced to reveal the identity of a prophesied Jedi queen, the Skywalkers' relative, Allana Solo, to convince her Tribe that her loyalties remained with them. She was made a Sith Saber, and then forced to open an attack on Solo. Soon afterward, Abeloth captured Vestara and Ben Skywalker in Ship. Abeloth attempted to convince them to join her in forming a new family of the Ones, the powerful deities of whom Abeloth was the last surviving member, but they killed Abeloth while her other manifestations were defeated, resulting in her destruction. Knowing Ben would not forgive her betrayal of Solo, Vestara fled in Ship, who promoted her to Sith Lady and took her to join another faction of Sith.