Kuwei is described as having black hair that falls on his forehead, a wide brow, the gold eyes of a Shu. He's sixteen and young looking. There is an extended period of time where Wylan looks exactly like him and it is only possible to tell them apart by the way they speak.
Kuwei is hard to get to know, lying about how well he speaks Kerch and imitating Wylan, tricking Jesper into kissing him. He plays dumb as a way of entertaining himself and annoying Jesper (who he appears to have feelings for). As well as tricky, Kuwei is clever, spending time trying to recreate jurda parem and drawing in his journals.
Kuwei's father, Bo Yul-Bayur, tried to make a drug that would mask Kuwei's inferni powers. Instead he created the dangerous drug jurda parem and, fearing it would be used as a weapon by his government, sent samples to Kerch, asking for asylum for him and Kuwei. Before they could be relocated to safety, Kuwei's father died and Kuwei was attacked by the Drüskelle. Kuwei was kept prisoner in the Ice Court, and was forced to try and recreate the drug until he was broken out by Kaz Brekker's gang.