^ They have never ever referred to themselves as a male either yet you'd refer to them as a "he". While even Danbooru, the site that's okay with Nazi joke arts, has the basic decency to tag works with Mizuki as 1other.
^ You don't assume someone to be of one gender when you don't know. You use they/them, it has been the gender-neutral 3rd person pronoun since 14th century and is not non-binary specific.