The Rise of Skywalker makes a major addition to Star Wars lore with the Force dyad, but simultaneously creates a plot hole for Kylo Ren. From the very beginning of the Star Wars sequel trilogy, Daisy Ridley's Rey and Adam Driver's Kylo Ren have a connection. In The Force Awakens, they seem to be two sides of the same powerful coin - one fighting for good, and the other for evil. The Last Jedi develops their bond into something more unique, as Rey uses the Force to commune with a topless Kylo across great distances. Finally, The Rise of Skywalker reveals the connection as a Force dyad - a rare and mystical occurrence whereby two Force-sensitives are halves of a larger whole.
Rey and Kylo Ren's Force dyad is explained to the audience in rather dramatic fashion. After Chewbacca is captured by the First Order, Rey and her friends steal aboard Kylo's Star Destroyer to rescue their lost Wookiee. The mission is a success, but Rey's eerie magnetism towards Kylo Ren draws the pair together once again, this time in the ship's cavernous hanger. Before the Millennium Falcon arrives to whisk Rey away, Kylo shockingly reveals that she's the granddaughter of Emperor Palpatine. He also slips in "what Palpatine doesn't know is we're a dyad in the Force, Rey." This is the first time Star Wars directly addresses the nature of Rey and Kylo's strange mutual obsession.
But how did Kylo Ren actually learn this information? As the Darth Vader wannabe himself states, Palpatine is unaware that Rey and Kylo form a Force dyad. This is confirmed in the final battle, when the reborn Emperor is shocked by the discovery and changes his masterplan in response. Since Palpatine didn't recognize a Force dyad developing between his only student and his own granddaughter, it seems his knowledge on the topic was patchy at best. Snoke was Kylo Ren's Sith master, but couldn't have taught his student about Force dyads because The Rise of Skywalker reveals the Supreme Leader is Palpatine's puppet. Some might argue that since Kylo Ren was part of the dyad, he intuitively felt what was happening, but then surely Rey would come to the same conclusion without her long-distance boyfriend mansplaining it.
That Kylo Ren alone knew about the Force dyad feels like a plot hole in the Star Wars sequel trilogy, as there's no obvious way he could've uncovered such information. Following the visions he and Rey shared in The Last Jedi, perhaps Kylo started digging into old Sith lore behind his master's back, attempting to work out what this mysterious connection could be. Alternatively, he might've shared a quiet word with one of the Sith Eternal cult members on Exegol, and learned about the Force dyad secretly through them. There are feasible ways Kylo could've studied up on his Force dyad knowledge, but The Rise of Skywalker doesn't address the issue, and this is symptomatic of a wider problem with the film.
Despite being the concluding chapter of the Skywalker saga, The Rise of Skywalker introduces a host of fresh concepts to Star Wars lore - the Force dyad, Rey Palpatine, the Emperor's resurrection, the Final Order, etc. But rather than properly explaining these elements and where they come from, the audience is left to fill the blanks for themselves. The Force dyad is a concept that could've added a fascinating new dimension to the Star Wars universe, but when Kylo blurts out the exposition with no logical explanation as to how he gained such knowledge, it's impossible not to view The Rise of Skywalker's Force dyad as just a convenient plot device.