I already talked about this a lil in the tags of my sexuality post but my thoughts are complex so I feel the need to really dive into what I believe with Kaz and his gender expression and why.
First of all: pronouns. I feel like Kaz uses he/they/it.
He because he aligns himself with masculinity. He/him is his preferred and what he uses.
They is also one of the preferred ones. He/they is fine, he/him or they/them is also fine, they don't really mind.
And Kaz likes it because it feeds the whole "what is he? Is he even a person? Is he a demon?" It makes him seem less human, more demonic, and he revels in it.
Kaz definitely goes for a more masculine vibe, but honestly? They also aligns themself as other, as demonic. They're not really anything like all of these people. Even his dark mercher suits make him look like a dark stain on the city, a nightmare manifest. Sure, he looks respectable, but the eyes, the corpse-like face, the "sharp glass boy of deadly edges" thing he has going on, makes him look more subhuman, almost undead. Which fits his whole Kaz Rietveld died in the harbour and was reborn as Kaz Brekker thing.
Gender wise, I don't think they labels themselves. They lets people talk and theorise. "He's obviously a man" "They're nothing like us" "Have you seen it?? That's not even a person!" When one of the crows asks they just tells them something theatrical like "I'm the demon under people's beds, merchling" "I'm the thing in your closest, Nina dear" "I'm people's nightmares made manifest, the personification of this city. I am crime made a person, I am horror in the flesh." But they never actually answers the question. They're not really sure. He knows that he uses he/him pronouns the most, alongside they/them, but he doesn't feel like a boy, not really. He hasn't really ever been like the other boys, even as Kaz Rietveld, much less as Kaz Brekker. Always a little odd, a little more on the quiet side. Analytical, searching, constantly watching. Unnerving.
He doesn't mind terms like boy and man but prefers demon and monster because he's silly like that and it gives him a rush. I feel like he'd hate the word lad.
I also think their gender identity is explicity linked to his rebirth as Kaz Brekker. They created themselves, almost entirely from scratch. And so they pondered it, thought it over. They decided that they liked it to an extent but only in certain scenarios. He uses he/they interchangeably, but also not.
As Dirtyhands Kaz sees themself as it. A separate persona. As Kaz Brekker, it's they/them but he only associates the two because they only realised they liked they/them after the rebirth. As Kaz Rietveld it's he/him. It's another way of separating himself into different parts. But Kaz Brekker is still Kaz Rietveld, no matter how much they try and deny it. Kaz Reitveld isn't really a persona the same way Dirtyhands is. It's who he is. Kaz Rietveld is a part of Kaz Brekker, and Kaz also uses he/him alongside they/them