Revan
this ancient Jedi Knight was one of the many which participated in the defense of the Old Republic during the Great Sith War, some 4,000 years before the Battle of Yavin. Revan and his fellow Jedi Knight, Malak, put down a Mandalorian uprising in the Outer Rim during the war, but were brought into contact with the Dark Side of the Force. Revan began to study the Sith lore, and eventually named himself Darth Revan, a Dark Lord of the Sith, with Malak as his apprentice. The two set out to wage their own battles against the Jedi Knights and the Old Republic, until Revan was captured in battle. Malak took up his master's mission and set out to continue the war against the Jedi. Meanwhile Revan was given a complete memory wipe by the Jedi Council, which eliminated the taint of the Dark Side and returned him to the ways of the Jedi. To ensure the transformation was complete, Revan was paired with Bastila Shan on a mission to locate Malak and the Star Forge. After the pair was captured, along with Carth Onasi, and held by Darth Malak aboard the Leviathan, Revan was given information about the truth behind his return to the Light Side of the Force. He couldn't comprehend the duplicitous act, and nearly fell in battle during an escape attempt. It was Bastila's brave attack that deflected Malak's attention, allowing Onasi and Revan to escape aboard the Ebon Hawk. Eventually, the group managed to destroy the Star Forge and Darth Malak with it, but the Jedi Order continued to struggle with schisms in its ranks. Revan himself recognized the nature of the galaxy, and left the Jedi Order to take the fight to the Sith. A large number of Jedi Knights followed Revan into battle, abandoning their training to fight the growing evil. Over time, the Jedi Order lost touch with Revan, and he was never seen again.