There's this really cool character in the Arc of a Scythe series by Neal Shusterman who is genderfluid. The character is a sailor, and they explain how when the light hits them in different ways, they change genders. Personally, it was one of the coolest, most powerful yet subtle ways of introducing a queer character. And it wasn't like this character was discussed once and never heard of again. No, like, this person is decently prominent in the book, and you see the other characters actively change pronouns and gendered terms. It was super cool, I loved it.