Obi-Wan and Rey
Anakin Skywalker became Darth Vader. He was manipulated, and he turned to the Dark Side, betraying all of us in the process. When the order was given to destroy the Jedi, Anakin killed all of our younglings in the temple. I followed him to Mustafar to try to talk sense into him and bring him back to us, but he wouldn't listen. He only saw me as someone who had betrayed him instead, and he fought me. I defeated him, but... I could not kill him. I too his lightsaber and I left him there. I thought he was going to die.