Vestara Khai is a human Sith apprentice of the Lost Tribe of the Sith on the planet Kesh, and the co-lead antagonist of the nine-book Fate of the Jedi series. She first appears in Christie Golden's Omen (Del Rey, 23 June 2009).
The Lost Tribe
Her people are the descendants of a Sith ship marooned on Kesh five thousand years earlier, who built a functioning civilisation in total isolation and were unknown to both the Jedi and to Palpatine's line. That premise — a Sith society that is a society rather than a two-person conspiracy, with schools, politics and family life — is the most interesting idea in the late Legends novels, and Vestara is how the reader gets inside it.
Ben
Her relationship with Ben Skywalker is the series' spine and it is genuinely well handled: she is deployed against him as an infiltrator, the attachment becomes real anyway, and neither of those facts cancels the other. Luke travels with them both for several books in the full knowledge of what she is, betting that she can be turned. The novels take that bet seriously for a long time.
She does not turn
The ending is the reason the character is remembered. Offered the Jedi path with everything arranged in its favour, she chooses the Sith — not through a lapse or a manipulation, but as a decision. Legends ended before the consequences were explored, which leaves her one of the more conspicuous loose threads the 2014 continuity reset cut.