Even though Obi-Wan Kenobi once told Luke Skywalker that it was next-to-impossible for a Sith to leave the Dark Side, Darth Vader redeemed himself after he gave up his life to save his son. This moment proved that, not only was there always some good left in him, but a Sith was never beyond redemption. (Except Palpatine of course).
Because Obi-Wan was so adamant that Vader was beyond saving, one would assume this is the only time a Dark Force user had returned to the Light Side. But there have been many Sith over the millennia who abandoned the Dark Side after learning the folly of their ways. Sith have turned good after they found love, were inspired by their family, or because they discovered they were being manipulated. There are Sith who have flip-flopped between good and evil many times; there are Sith who never conducted a single act of evil; there is even one darksider who returned to the light long after he died.
Even though some of these examples are not considered canon, (especially the Star Wars Legends series), we are going to look at every single Sith that found their way back to the light.
20. Jaina Solo
In the Star Wars Legends novels, Han Solo and Leia Organa had three children - Anakin, Jaina, and Jacen. Because the blood of Darth Vader ran through these children's veins, it was no surprise when not one but two of them turned to the Dark Side.
Jaina was held in high regard due to her skills as a Rogue Squadron pilot and her habit of always fighting on the front lines of battle. Luke Skywalker was so impressed with his niece, he gave her the title, Sword of the Jedi.
During a mission, Jaina and Anakin were tasked with eliminating a creature designed to hunt Jedi called the voxyn. When Anakin was killed, Jaina was left vulnerable, allowing the Dark Side to seep into her. Consumed by rage, she vowed to stop at nothing until all her enemies were wiped out. When her comrades saw Jaina was willing to risk innocent civilians to get what she wanted, a former Sith called Kyp Durron stepped in and convinced her to let the Dark Side leave her.