Darksiders Who Turned Away From The Dark Side
The Jedi and Sith have a long history as adversaries at best and enemies in war at worst. One fighting for the light and the other a harbinger of darkness. We have heard numerous stories of proud Jedi warriors and wise masters falling to the darkness, becoming Sith or other sorts of darksiders. However, we don't hear a lot about those on the dark side being shown the light and renouncing their dark ways. It may not be as frequent as the former, but it has happened a lot in the Star Wars lore. Several darksiders have fought through the darkness to redeem themselves and become great Jedi.
Darth Sajar
During the Great Galactic War, the Sith control a lot of power throughout the universe. During this time, Sajar is both a Sith Lord and member of the Dark Council. Around this time, there is a Jedi master named Tol Braga, who takes up a strategy of trying to convert the Sith to the light side of the Force as a means to combat the Sith. Darth Sajar, trying to set up a trap for Braga, leads him to Dantooine. The two battle for several days, both making arguments for their respective side of the Force. In the end, Braga convinces Sajar to renounce the dark side, and Sajar became Tol Braga's padawan.
Starkiller (Galen Marek)
The son of a fugitive Jedi during the Great Jedi Purge, Galen Marek is born into hiding until Darth Vader finds him on Kashyyyk. Vader senses the power in Marek already within him at this young age, and Vader takes him on as his apprentice. Later on, Vader betrays Starkiller, which leads him to join the emerging Rebel Alliance. He also takes on Jedi Master Rahm Kota and learns the ways of the light side of the Force. In the end, he (depending on choice) sacrifices himself for the Rebels.
Darth Revan
Darth Revan has, to put it simply, a complicated life. He starts out as a young Jedi prodigy and leads several Jedi to victories during the Mandalorian wars. However, Revan would later turn to the dark side to become the Sith lord Darth Revan. He and Darth Malek would wreak havoc on the Jedi as two of the most powerful Sith at that time. Then he loses his memory when captured by the Jedi. They use the Force to turn him back to the light side in hopes he would aid them in fighting Darth Vitiate.
Darth Vader (Anakin Skywalker)
Anakin Skywalker is a powerful Jedi Knight, and he is said to be the chosen one that would truly bring balance to the Force. However, he is seduced by Darth Sidious to turn to the dark side, leading him to betray the Jedi order and become Darth Vader. Much later on, his son, Luke, senses the good in his father and tries to bring him back to the light side. Eventually, he succeeds, and Anakin turns on the Emperor and sacrifice himself to save his son. This completes his return to the light side.
Mara Jade
Mara jade is trained from a young age to be an assassin for Emperor Palaptine. Before his death, he issues his last command to her: to kill Luke Skywalker. She is unable to fulfill the command and eventually works alongside Luke. As he did with his father, Luke senses good in her. As they worked together more, Luke teaches her about the light side of the Force. Eventually, she rejects the dark side and becomes a Jedi. Mara Jade and Luke Skywalker marry later on.