Not sure I’m wording this right but I’ll try so most of the plot twists in the six of crows duology that have a large impact on the overarching plot are revealed to actually have been controlled by Kaz, such as “He was just Wylan Van Eck. He told them everything”, Inej being caught by the Ice Court guards and Heleen, the fake Council of Tides, and so on and so forth. One of the only major twists not controlled by Kaz is Kuwei’s existence and presence at the Ice Court in the place of his father. And one of my favourite things about this at that it so massively relies on perception, as is so symbolically relevant in many themes of the novels but particular in the theme of classism and how each “side” of Ketterdam views each other and themselves. Because yes, this is a plot twist that was not pre-planned by Kaz and that he did not figure out in advance of being confronted with it (he internally expresses hope that Kuwei is “a surprisingly young Bo Yul-Bayur” and not just some “hapless prisoner that Nina and Matthias decided to liberate” when they fell the ash, and later asks Nina to explain why the “illustrious Shu scientist looks like one of Wylan’s school pals” please note I’m quoting from memory there may be some mistakes and/or imperfections), but even though this is true it’s also true that in the literally third chapter Kaz immediately and correctly theories that Bo Yul-Bayur is dead. Van Eck states that they have sources telling them he is alive, and Kaz willingly accepts this - even using it to convince Nina when she raises the same assumption of Yul-Bayur’s death. This is a minor example, though of major importance, of Kaz’s ultimate failing grace in the first novel: he was willing to accept Van Eck at his word. We see him criticise and bully himself for believing Van Eck throughout the ending of this book and through Crooked Kingdom, particularly in the way he blames himself for Inej’s abduction, but that’s for the larger, intentional lie; the money. What we see in Kaz’s acceptance that Yul-Bayur must be alive is him tossing his instincts aside in response to Van Eck, essentially because he trusts the rich, respectable mercher to be honest, and to have no motive in lying to him; just as the Dregs (specifically mentioned) and the rest of the gang members (strongly implied) in Crooked Kingdom assume Kaz’s guilt because they can see no reason why a rich, respectable mercher would make false accusations like the abduction of his son. After the original assertion in chapter 3, we receive various pieces of evidence that we and the characters both take to renforce Van Eck’s claim, such as the other teams moving on the Ice Court, most notably the Shu, and (the most damning) Brum’s references to “the scientist”. And yes, if Yul-Bayur had been alive then these would all count towards evidence of that. But the tiniest hint of doubt, that you barely even register as doubt in the moment, comes from Brum when he describes “the scientist” as being “stubborn” for “still mourning the death of his father”. Matthias’ internal response is that he knows nothing about Yul-Bayur’s father so the reader is from that moment prepared to discover a mystery about the elusive father, not the unknown son. From the second we met Kuwei, the fate of the job and of Van Eck’s being a liar was sealed in an absolute excellent piece of foreshadowing. Because Kaz ignored his clear instinct, that Yul-Bayur was dead and something else was at play if the Fjerdans and Shu both truly had parem, in favour of believing Van Eck’s word. And Kaz turned against his instincts, that he shouldn’t take the job because it was a suicide mission, in favour of what he openly describes as his own greed by saying that the lever was working in Van Eck’s favour.
So I think what I’m trying to say here is that Kuwei’s appearance has a two-fold relevance as a plot twist: first in the subversion not of Kaz controlling the narrative but still having predicted some aspects of this, and second as foreshadowing of another major plot twist that Kaz had not control of, that being several aspects of the ending of the book. I hope this made sense, I’m very tired and I’m not sure that I’ve articulated this correctly, but i thought it was interesting and these posts tend to work better when I just put all my thoughts onto the page as they come instead of trying to structure it or whatever