Starkiller's Perfect Clone, 'The Dark Apprentice'
As we alluded to before, the line of cloned Starkillers in The Force Unleashed lore didn't actually end with the betrayal of Subject 1138. But depending on how players of the video game decided to take vengeance on their Sith Master, they may or may not have even known of this next, more perfected clone. Dubbed the 'Dark Apprentice,' this student actually was everything Vader hoped he could be. All the powers of Galen Marek, but none of the weakness.
During the final act of the game, players rose to face, and ultimately defeat Darth Vader in lightsaber combat as part of a Rebel victory. If the player chose to embrace the Dark Side and slay Vader, then the Dark Apprentice intervened with a lightsaber through Subject 1138's back. And the clone, like the player, learned that the Dark Apprentice had become Vader's perfect weapon (yellow Sith eyes and all). Having seen that very fate in a vision, though, the canonical ending of The Force Unleashed II sees Subject 1138 stay his hand, and live. But either way, the Dark Apprentice lived... to do what has not yet been revealed.
Antinnis Tremayne, Sith Inquisitor
If you're looking for proof that not every recruit of the Jedi Order was a devout believer, merely a kidnapped, Force-sensitive baby, then Antinnis Tremayne is definitely going to help your case. Fortunate to simply be taken prisoner, and not outright executed in Order 66, Antinnis was deemed of particular interest by Imperial Intelligence. So when it came time for Vader to see if the political prisoners showed spunk, Antinnis knocking of Vader's breathing apparatus convinced him. He then killed the others who disappointed, but for Antinnis, it meant a promotion to the first member of the Inquisitorius.
Charged with both hunting down rogue Jedi in hiding, and recruiting potential Force-Adepts into the ranks of the Inquisitorius, Antinnis would eventually prove effective enough to become High Inquisitor. And even when his tussles with Jedi cost him his right arm and the right side of his head, Antinnis Tremayne proves an important point: falling to the Dark Side of The Force can be easy, if you're more than happy to have Darth Vader as your boss, and mentor in the ways of the spirit and saber.
Rillao & Hethrir
In Antinnis' mission to hunt down those Force-Adepts whom he suspected of secretly aiding Jedi survivors of Order 66, he came across two would end up playing a much larger role in the galaxy's story. Unfortunately for the Firrerreon named Hethrir and his mate, Rillao, it wasn't until after Antinnis had tortured them that he decided to offer them an invitation into the Empire. As luck would have it, they accepted! But when the pair were brought to learn from Darth Vader himself, Rillao's unique attunement to the healing aspects of The Force made the Dark Side seem intensely unappealing. She fled with the pair's unborn child, but Hethrir's path up the Empire's chain of command went a much different way.
The pair's story begins in the Star Wars novel The Crystal Star, and Hethrir wastes little time in proving he had the constitution to not only survive in the Empire, but thrive. The Empire viewed non-humans as lesser creatures? Hethrir unleashed a virus to wipe out his entire planet. And if his ascent to the role of rigid and 'just' Procurator of Justice didn't get his cruelty across, how about building his fortune selling 'body-wood' - trees resembling human flesh? Hethrir's failed efforts to create a new Empire after Return of the Jedi aside, he sure made Vader and Palpatine proud.
It's no secret that Flint was created to offer a glimpse of the life Luke Skywalker could have had, assuming he followed through on his mission of joining up with the Empire. As the son of a Jedi Knight slain by General Grievous at the end of the Clone Wars, Flint's first meeting with Luke Skywalker was that of a young man meeting his idol... until Luke dismissed him, and Flint grew disillusioned. Pairing that moment with witnessing his mother's death, and Flint came to embody Anakin more than Luke. As a result, his explosion of emotion popped him onto Darth Vader's radar.
Taking Flint under his wing and recruiting him as one of his Stormtroopers, Flint left behind the white helmet and armor of the Empire's shock troops for a greater calling long before Finn in The Force Awakens. Flint eventually was called up to become Vader's new apprentice, mastering the ways of The Force in a larger plan to compete with Lumiya, to determine the true apprentice worthy of ascending when Vader killed the Emperor. Alas, it wasn't to be. Fortunately for Flint, his pull to the Dark Side was shaken when Vader fell, and he would eventually return to the side of the Rebels before dying in action.
Those are the known apprentices of Darth Vader, but who knows how many more might be added into the NEW Star Wars canon? Whether they're Force Users recruited to become true Sith Lords, or Force-sensitives brought under Vader's cape to help fortify his Empire, each of them is deserving of more time in the spotlight. And if Disney is rethinking their plans for future Star Wars movies, maybe spending some time on heroes and villains that audiences haven't met is a smarter path forward?