Nephthys
The Gr8est!!!!!!!!
In the 1st novel.
'"I don't hate you," the apprentice went on, blocking him blow for blow. "I pity you." With a new strength of his own, he forced Darth Vader onto his back foot. "You destroyed who I was and made me as I am now, but this wasn't your idea. It was the Emperor's, and it's what he's already done to you." A strip of Darth Vader's cape fluttered away, smoking. The two came closer together until they were face-to-mask. The apprentice stared directly into the black eye guards of his former Master. "You are his creature just as I was yours-but you've never had the strength to rebel. That's why I pity you. I will no longer serve a monster, and if I have my way I'll make sure you don't, either."
Vader tried to pull away, but the apprentice followed him, keeping him on the back foot.
"I will kill you," he said, "to set you free."
The lightsabers flashed again-and it was the apprentice who found the chink in the armor that both of them had been waiting for. Vader's lightsaber moved too slowly to block a blow to his chest, allowing the apprentice's blade to slash deeply across his armored throat. Vader staggered backward, gloved hand upraised to the smoking wound.
There was no blood. Instead of pressing the attack, the apprentice stood his ground. Despite himself, he was as surprised as his former Master clearly was.
For a moment, the only sounds were the twin humming of the lightsabers and the wheezing of Darth Vader's respirator.
Then the Dark Lord laughed.
It was an awful sound, empty of humor and full of mockery. In it, the apprentice heard a decade and a half of torture and abuse.
Anger flared. He lunged forward. His former Master barely blocked the blow. A second scored a deep wound across his black-clad shoulder. A third stabbed deep into his thigh.
Darth Vader reeled backward, servos whining in his injured limbs and lightsaber shaking.'
After that he pwns him with the Force, but its apparent from the above that he's better than Vader in lightsabers too. Theres also a bit in the game where he owns him in lightsabers.