Searching that page for the word witch only found matches on your question and my single comment, which was not replied to. Reading through all of that question and its answers as they stand now doesn't necessarily tell the whole story, but I feel a bit awkward now. If I set up enough cognitive dissonance in your mind you felt like you needed another question to address it, I'm very sorry. This was not generally where I was going with my comment, although I considered it a possibility. But I was more thinking in terms of admiration, respect, and appreciation - Granny Weatherwax's gender and orientation would never come up, unless someone were asking for guidance that was related to it, because she was feared and respected. As far as we know, she was a straight cis woman, but apart from Discworld making it clear witches were supposed to be women, a difference in her gender or orientation wouldn't have mattered to the story significantly. One of the books would've needed some different pronouns.
There are other notable witches that I could comment on, but I don't think any of them have quite Granny's stature. Locasta-Tattypoo from the Wizard of Oz probably wouldn't change at all due to a difference in gender or orientation. Other than her pronouns, I don't think we get any insight in that direction at all with her. Glinda, also from the Wizard of Oz, on the other hand... we still don't get any insight into her gender or orientation, but we do get a significant vibe that she is loved and appreciated enough by the good people of Oz, and feared enough by the not good people of Oz, that we can be confident her gender and orientation wouldn't be any kind of an issue for her socializing with people.
Speaking about Kem, for their gender or orientation to be mentioned in the story, it should matter somehow. But how exactly that matters could be managed in a great many ways. For example, it could just be that Kem has difficulty identifying with gender binary people. Or maybe they just confuse people, but not in a manner that antagonizes people. It could be that they mostly focus on helping out other people of non-standard gender or orientation. It would feel a bit odd, however, if we learned that Kem was in some manner gender queer in the first paragraph, and it never had any impact on the story ever.
It could also be that Kem's gender and orientation are never an issue, because that isn't ever an issue in the world of your story. It would be nice if it wasn't an issue in this world. But since it is an issue for some people in this world, I think it's important for it to matter in some fashion in your story for it to be a detail worth mentioning in your story. But it's your story, not mine. I only respond because you asked and I like to be helpful.
You don't need to include elements in your story that you want to not be in your story. It's your story. Your story would likely be more interesting to a wider audience if you have some form of opposition. But that opposition could be whatever you want it to be. It also doesn't have to be opposition to Kem specifically. I've read many stories that were just the main character helping other people with the things that gave those other people problems.