In the Setsubun event Tomoe says something along the lines of “Worry not! I am aware of modern visions of chastity!! So we’ll devide the hot springs into make and female ones, and give a third option for people of a different gender,” and it’s a direct parallel to how she sees herself. She’s aware that people divide the world into “oni” and “humans,” just like most people divide gender into “men” and “women,” but the same way some other characters just don’t vibe with that gender thing, Tomoe… doesn’t get the whole oni/human thing. She knows that for onis, her inability to grasp their way of living makes her incomprehensible. For humans, she’s a monster from her horns and her strenght alone. Herself doesn’t know where she stands. She can’t just shave her horns and be human, but she doesn’t wish to be an oni either. Ultimately, she’s at her happiest when viewed as neither, and upon realizing that, decides that this is how she’ll be from now on.