Board Thread:General Discussion/@comment-24100805-20160201152915/@comment-32182236-20180315200111

Yeah, they have no canon gender. "They" is used because nobody knows their gender.. After all, we never reveal it to anyone. So, why WOULDN'T they use "they"?

Hence, "they" isn't evidence that Frisk MUST be non-binary. It just means it's a possibility. He's whatever gnder you want her to be. (Yes, I used both pronouns on purpose. The whole point is they're both correct.)