Alex I view as being bigendered though probably definitely not getting that that is what they are. They would definitely work. But she and he would also work depending on where Alex's gender is at the moment. I imagine they'd be a brat about this to people. Like openly refusing to answer certain questions without being a shit. Granted the "are you a boy or a girl?" question is pretty worthy of being a shithead about! Also I like the idea of Frisk not really noticing what other people complain about where it regards Alex's behavior.