It's so funny Matthias' chapter meeting Kaz for the first time, he look at him for the first time and is like:
A boy wearing a guard’s uniform stood before him, dark eyes glittering, a pistol in one hand, a walking stick in the other.
A BOY. Matthias look at him and sees a boy. Then, a few moments later he is like
“They’re going to be too busy to check,” said the strange, pale boy.
Kaz is a strange boy now. Still a boy. And then shortly after he is
He looked up at the pale demon with his black-gloved hands.
PALE DEMON HWHAUDHWUDHA Kaz only needed a few moments to go from being seen as a boy to being seen as a demon. THIS MAN.
Later in the chapter Matthias even calls Kaz a creature who had spent too long in the dark, KAZ IS ALL DARKNESS
Matthias listened silently to the strange words coming out of the pale, freakish boy’s mouth. Matthias knew monsters, and one glance at Kaz Brekker had told him this was a creature who had spent too long in the dark—he’d brought something back with him when he’d crawled into the light.
Of course, Matthias is superstitious and this is why he is all this boy is a demon, but this is also the reason why it's so funny to see Matthias meeting Kaz Brekker for the first time.