Introduced in the 1991 novel Star Wars: Heir to the Empire, in the Expanded Universe continuity, Mara Jade was originally the “Emperor’s Hand,” a special Force-sensitive agent that Palpatine sent on key missions.
As the books passed, Luke helped Mara hone her Force abilities, and the two eventually married and had a son named Ben.
Sadly, Mara was killed by her nephew, Jacen Solo, in the novel Sacrifice after she learned he’d become a Sith.
However, this was primarily driven by a telepathic command Palpatine had driven into her head before he met his demise, and Luke was able to break Palpatine’s hold over Mara.