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One way to keep it from taking over your story is to make it unexceptional. Quite literally. Kem is nonbinary. If Kem, other characters, and the narrator don't make a big deal out of that, don't...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/41947 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
One way to keep it from taking over your story is to _make it unexceptional_. Quite literally. Kem is nonbinary. If Kem, other characters, and the narrator don't make a big deal out of that, don't either hide it or gawk at it, and just go about their lives, you'll convey the message that _this is normal in Kem's world_. That part about not _hiding_ it is important, though. If you introduce a nonbinary character and that fact is then _invisible_, you risk looking like you only threw in that detail to check off a box and score points. Kem is nonbinary and that affects how people interact with and talk about the character. That has to be _present_, but it doesn't need to be _central_. Treat Kem's gender the same way you treat other character's formative traits. A character who grew up in a broken home doesn't talk about the fights and divorce all the time; a character who's a genius doesn't only talk about being a math prodigy; a character who's of small stature doesn't always point that out. But the first one might have a strong flinch reaction to arguments, the second might always be working on some abstract problem in an ever-present notebook, and the third might be seen often stretching to reach things, preferring platform shoes, or leaving top shelves empty. Kem is nonbinary and that _affects Kem_; figure out how and show that. I don't know enough about nonbinary gender to have specific advice about all the ways (beyond pronouns) that it affects the person's life. If you know, then look for ways to show it akin to what I've described with other traits. If you _don't_ know, then try to find out before you write your story, so you can write a three-dimensional character rather than a caricature.