Right now Yuusha is stirct, assertive, arrogant/proud (for thinking she can take care of and punish all the students), and justice-minded. She has a good heart, but some people may not take her help well if they believe Yuusha doesn't have the right to do that. Other than that Yuusha's character is lacking in detail and could be assisted by adding more qualities to her character.
Yuusha's backstory may also require more description. What I got out of it was that Yuusha's parents died, she was raised by her grandfather and grew up with her cousin, and she grew to be a great kendo practitioner. It explains why Yuusha is the way she is now, which is good but other than that it's still in need of more work.
Maybe have some moments where Yuusha's beliefs are challenged and she's forced to rethink her ideals? Or maybe she is more stubborn and doesn't deal with those things often?
Her appearance is something I usually don't see around here, so it kind of sticks out.
Her personality is something I would expect. A person who means well going out of their way to help others and wants to be this knight in shining armor. It could have something special to help her stick out. The background is something I see often with OCs where someone has a tragic past. Luckily, Yuusha's is definitely not too tragic, so it's pretty much fine. Again, a lack of information is typically the problem here.
Her backstory is okay, but it doesn't have anything special going on for it. However, it is not overdone on any tragic details, which I definitely approve of.
The article is definitely alright for reading! There's some minor mistakes here and there, but I could almost completely understand what the writer was trying to say.
Some mistakes include:
So on and so forth. I honestly did not notice some mistakes when I first read through the article. They are easy to miss so they aren't a big deal, but they should still be ameliorated.
The structure of some sentences requires attention.