Sidenote, I both love that the Knights of Ren are all just unused Jedi Killer concept art-- you can see the helmets to the extreme left and right in the post above-- and I think it is a real shame these guys just get utterly wasted in The Rise of Skywalker, but I don't think they really needed to do anything in that film, either. There's a wonderful tradition of characters suggesting so much more story in Star Wars, and I think just a little clearer framing of these dudes early on would have made them feel a lot more fleshed out.
Because like, these dudes don't do anything in The Empire Strikes Back, but we love them all. Dengar, IG-88, Bossk, 4-LOM and Zuckuss-- all they do in Empire is stand there, look pretty, and apparently fail their assignments offscreen. They aren't named, never return in a subsequent scene or film; apart from Boba Fett, they just evaporate as far as filmed Star Wars is concerned.123
Star Wars actually is great when it just throws dudes like this away. They just need to be in frame for a minute so we can memorize their faces first. And maybe don't slag them in the finale just to tie up loose ends.
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Except for Bossk, who enjoyed a brief return to canon in The Clone Wars as young Boba Fett's crime uncle.
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And while IG-88 never returned, IG-11 was one of the best characters in The Mandalorian S1, and one of the best vehicles in S3.
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And Dengar is a recurring villain in Lego Star Wars: The Freemaker Saga