Kai: Hufflepuff. Kai is driven by duty above all else. He will always choose what is best for his people, putting the greater good above his own interests. Everything from his personal safety to his love for Cinder comes second to stopping the letumosis plague and making peace with the Lunars. However, while that could all point to Gryffindor, I don’t think so. He prefers to compromise, negotiate, and try to keep the peace rather than stand up to the evil in his world directly – and yet, unlike a typical Slytherin, he is not ambitious, instead viewing his role as a leader in a very altruistic way and thinking of himself only after his people’s safety is ensured.
Wolf: Hufflepuff. Wolf’s story is all about loyalty. The only reason he is able to leave his pack is because his loyalty to Scarlet is stronger than his loyalty to them; when the two conflict, he chooses to betray them in order to protect her. Once Wolf’s loyalty shifts to Scarlet, it changes on a larger scale as well. He becomes a member of the Rampion crew, helping and fighting for Scarlet’s friends just as fiercely as he had once fought for the Lunars.
Thorne: Gryffindor. Thorne isn’t a perfect fit for Gryffindor. He’s bold, adventurous, and unafraid of danger, but unlike many Gryffindor types, he doesn’t have their usual tendency toward selfless heroics. Which isn’t to say he never risks his life or acts courageously, because he definitely does. It’s easy to rule out Ravenclaw and Hufflepuff, and while he has some Slytherin traits, he lacks their ambition. He seems to enjoy living in a spaceship and jumping from one dangerous situation to another, which is definitely the sort of attitude only a Gryffindor would have.
Jacin: Slytherin. Jacin isn’t concerned with rebellion or justice or any of that. His one and only priority is keeping Winter safe, and he’ll do just about anything to accomplish that, including working for and betraying both sides. And his plan to fake her death is both cunning and brilliant. “Those cunning folk use any means to achieve their ends” is a good description of Jacin’s single-minded determination to protect his princess and willingness to use any methods, right or wrong, to do so.