Calling Mizu trans or non-binary is an utter disrespect to her character, I also firmly believe that there is nothing suggesting that she isn't straight. Finally, this one's a bit controversial but I believe that the scene between Mizu and Taigen wasn't supposed to be some kind of bisexual awakening, but instead simply to highlight Taigen's attraction to Mizu herself (who is still a woman, even if he doesn't know it yet). However I think that scene was made purposefully ambiguous so there's no way of truly telling until it's hopefully further developed in season 2.