When Geoff Johns rewrote Hal and Sinestro's first meeting in Rebirth, the latter mentions having trained several other Green Lanterns before Hal, which is just a fascinating concept to imagine.
Green Lantern: Rebirth
Stel is easily the most recognizable name among those that Sinestro lists here, and he still occasionally appears in the comics. You might also know Arkkis Chummuck if you're familiar with Corps history, though he's often mistaken for fellow Toomeyan Barreer Wot. Khen-To is the most obscure one, as his sole appearance is in Emerald Dawn.
Anyway, these three guys being established as Sinestro's students is a bit of a retcon, since it was never mentioned previously who else Sinestro had trained besides Hal. Which in itself is a retcon, because originally Sinestro had been expelled from the Corps before Hal ever joined. It wasn't until Emerald Dawn II that their current mentor/protege relationship was established, as well as the fact that Sinestro had recruited and trained other Green Lanterns.
History of retcons aside, I do wish that line about Sinestro's other students wasn't just a throwaway reference. The Corps is at its most interesting when the members have connections and relationships with each other. I get why no one, not even Johns himself, expanded on Arkkis and Khen, the former was killed off back in the 80s and the latter only existed to die while defending Oa in Emerald Dawn.
But Stel! He's still alive and kicking ass! There's still so much you could do with that connection to Sinestro (and Hal by proxy)!
Like, imagine what Sinestro's downfall might have looked like from Stel's perspective. He wasn't there like Hal was, he only learned of the atrocities committed on Korugar when the rest of the Corps also did. And upon learning of what his mentor did, he almost certainly went back through his memory banks, searching for any sign of whether Sinestro was always that corrupt or if he’d changed at some point. Trying to see if there was a point when Stel could have prevented all of this.
Or, if Stel isn’t the type of robot to dwell on the past, perhaps he seeks out Hal Jordan. Especially if Arkkis and Khen had already died by this point, who else could possibly understand what Sinestro's last student would be feeling?
One day, this human will become the greatest Green Lanterns, far eclipsing his traitorous teacher. But today, Hal is just a rookie who has seen his mentor put on trial and expelled. He is unsure of his place in the Corps, if there even is one for someone trained by such a treacherous monster. When a hulking android approaches him after the trial, Hal thinks that it's his turn to be thrown out. But to his surprise, the robot introduces himself as a fellow protege of Sinestro, and asks Hal how he's holding up.
This is one of the strangest conversations that Hal has ever had, speaking with an alien that looks like a sci-fi robot but addresses his doubts with understanding and compassion. Stel reassures Hal that the Corps will judge him on his own merits, rather than by the actions of Sinestro.
(that's Barreer, not Arkkis btw)
Even after Parallax and Emerald Twilight, Stel is willing to give Hal the benefit of the doubt- and is respected enough that his fellow Corpsmen will take his side over Salakk lmao