@Filene *nods* I've had similar mixed thoughts about writing stories in which I assign a gender to Chara Dreemurr, or (to put it more correctly) have Chara arrive at the conclusion that they in fact regard themself as female, as "she" and not as the indeterminate "they". Because that's what happened to myself, after all--while inhabiting the persona of Chara Dreemurr to speak through.

It'd be different with this Undyne idea, though. That'd be just a "wouldn't this be cool" as opposed to trying to reflect a genuine struggle over gender identity.