Maybe a good place to start is to get into specifics?
For instance, you say their parent(s) are a Japanese weather deity - which one are we taking here? The first one that comes to mind is Raijin, the Japanese God of Thunder (I think there's also technically a lightning deity that works in tandem with him - potentially they're her mom and dad)? but there might be more. Doing research can often lead to better clarity and hidden opportunities and connections you never noticed before.
With her direct parentage in mind, you might be better able to nail down her physical appearance. Off the top of my head, I imagine her hair could be very cloud-like and rise and fall over her shoulders in such formations; it could also function as a sign of her mood, in addition to her weather-based powers. If her hair is white and fluffy, she's happy. If its gray and stormy, with flashes of lightning, then she's obviously angry. A fine fog or mist might mean she's sad, depressed, or maybe just tired. And then of course, there are other monster features to consider, like pointed ears, fangs, claws, etc.
Just a quick thought, but you may want to change her first name so there isn't any confusion over what type of monster she's suppose to be, since actual kitsune are very different than what she sounds like.
You said that you have her mannerisms already figured out. This can actually help in designing her appearance, particularly in clothing - let it reflect her basic personality traits. If she's shy, then she might be bettered suited to wearing oversized sweaters, hoodies, or jackets that she can 'hide' in. If she's flamboyant or preppy, then her clothing might be brightly colored and comparatively skimpy (short-sleeved shirt, shorts, sleeveless dress, stuff like that). If she's aggressive or even artistic, she might have tattoos, piercings, or other means of expressing herself.
Lastly (at the top of my head, at least) for her basic story, why did she come to attend Monster High? Who would she most likely be friends with (it's best not to list the main group of ghouls for this, since that might come across as cliche or not very well thought out)? Obviously with this franchise, there's no dramatic reason like 'X killed my father, so I'm hunting them down in order to avenge him and restore my family's honor' but see if there's a better reason than 'X was lonely and wanted to make new friends'.
Does that help?