Remember, these are just my opinions.
Sophie: Gryffindor. She's reckless and really brave.
Keefe: Gryffindor. Just like Sophie, he's brave and reckless.
Biana: Gryffindor. Again, she shows a lot of bravery, especially at the end of Nightfall.
Dex: Ravenclaw. He's really smart and incredibly creative.
Fitz: Hufflepuff. He is loyal to his friends despite being a bit mean to Sophie at certain points. He's also really hardworking, and he does seem to value honesty, as shown through his cognate exercises with Sophie. I know honesty is required for cognates, but Sophie tends to avoid it, while Fitz is more open to it and clearly finds it important.
Tam: Hufflepuff. While he isn't incredibly trusting when he first meets people, he's very loyal to Linh and works hard to protect her.
Linh: Hufflepuff. She's very loyal to Tam and all of her friends. She's also worked really hard to master her hydrokinesis.
Stina: I'm honestly not sure. A lot of people say she's a Slytherin, but come on. She's not shown to be particularly cunning, ambitious or resourceful. She doesn't really have a personality; she's just a jerk. She started to get one in Neverseen, but in Stellarlune she went back to just being mean.
Timkin Heks: Slytherin. He's incredibly ambitious. I mean, no talentless person has ever been in the nobility, and yet he keeps trying. And no, I'm not saying he's a Slytherin just because he's mean.
Anyway, that's all for now. I'll probably make a part two of this, but I have to leave for a thing now so I'm ending it here. Let me know what you think!