Warning: SPOILERS for Star Wars: The Last Jedi are ahead!
When Ben Solo was seduced by the dark side and took the Kylo Ren identity, he didn't just become Supreme Leader Snoke's apprentice, and thus one of The First Order's most powerful agents. He also became the leader of the group known as the Knights of Ren. As far as we know, the Sith are no more since Darth Vader and Emperor Palpatine died in Return of the Jedi, but the dark side still looms in the galaxy far, far away, and the Knights of Ren have become the primary manipulators of this spectrum of the Force. But even with The Last Jedi giving Kylo Ren significant character development, mainly through him killing Snoke and assuming command of the First Order, his fellow Knights were nowhere to be seen in the latest Star Wars movie, leaving us wondering where they were and if they will appear in Episode IX.
So far our only glimpse at the Knights of Ren in the Star Wars movies came from Rey's vision during The Force Awakens. Upon touching Anakin Skywalker's lightsaber, she was bombarded with a flurry of moments both past and future. One of these moments began with a man being stabbed with a crimson-bladed lightsaber, and seconds later, we saw Kylo Ren and the other Knights standing near the slain warrior, with other bodies scattered around them. We never learned why this slaughter happened, and like Kylo, the other Knights of Ren are masked, so we don't know what any of them look like. However, The Last Jedi may have dropped a clue about where the other Knights came from. While divulging the truth to Rey about why Ben Solo fell to the dark side, Luke Skywalker mentioned that after Ben destroyed the Jedi Temple and killed most of Luke's other apprentices, he left with a select group of like-minded fellow students. This could mean that the other students are the other Knights of Ren, explaining how they're also strong in the Force. After all, if they were fine with their master and other "classmates" being taken out of the picture, it's not a stretch to assume that they would don black costumes and start using red lightsabers.
And that's all we have to work with concerning the other Knights of Ren as of The Last Jedi. None of the Star Wars books, comic books, video games, etc, have delved into their background yet, so they remain enigmatic. Given how The Last Jedi showed The First Order wiping out nearly all of The Resistance's forces, it's surprising that Kylo Ren's Force-wielding allies didn't play any part in spreading the carnage. It had been speculated that we might see Luke and Rey duel the Knights of Ren during the movie, but that was incorrect, and instead the big battle was Rey and Kylo fighting Snoke's Praetorian guards. Even though Kylo Ren was Snoke's main servant, obviously the Hugh Hefner-inspired villain must have found the other Knights useful, otherwise he wouldn't have kept them around. So at minimum, we can assume that the Knights are operating in other corners of the galaxy to enforce the First Order's will, but still, it would have been nice to have seen them, even if it was just the equivalent of another cameo.
We're now two-thirds of the way through this new Star Wars trilogy, leaving Episode IX as our last opportunity to meet the other Knights of Ren on the big screen. With Kylo Ren now the Supreme Leader of The First Order, it's possible that Kylo may start consulting with the other Knights more regularly. General Hux has a bigger axe to grind with Kylo now more than ever, so it would make sense for Kylo to start depending on others who share his vision and can use the Force. It's not like we're talking about a group that's the same size as the original Jedi Order, which had thousands of members. Looking back at Rey's Force vision, there are only six other Knights present. It's a little bigger than the Sith Order post-Rule of Two, but still a manageable size, thus reducing the likelihood of in-fighting (though treachery and betrayal are never fully off the table).
If the other Knights of Ren do appear in Episode IX, it would also be great if instead of just standing in the background while Kylo Ren remains in the spotlight, we would also see them in action. It's unclear how much time will have passed between The Last Jedi and Episode IX, so whether Rey has other Force-capable allies by the time the 2019 Star Wars movie unfolds or if she remains the last Jedi, it would be great to see her facing off against the other Knights of Ren before she battle Kylo Ren one last time. Episode IX needs to conclude not just this trilogy, but potentially the entire Skywalker saga, with a bang, and throwing in the Knights of Ren can help accomplish that. If you have six other dark side users at your disposal, it would be a waste not to use them.