Well, there you have a beginning.
She goes to school and has stuff too. What kind of stuff does she do in her daily life, besides helping her best friend? Does she go to the same class the MC goes to? How’s her family, her love life doing?
Once I read something like, treat your secondary characters as if the story was about them. Can’t remember where I read it or who said it. But it can help you here.
Other questions: Why does she support your MC? What are her motivations to do so? What does she get from it? Why does your MC need her help? What can she do for your MC that no one else can? What if she fails, what would happen to both of them? What does she think about helping your MC or is it a blind help?
To me this kind of character, the one that exists to fill a p
ossible unexisting void, with no story on their own, isn’t necessary. If you are not showing their point of view, their life, their problems and only breath because of the MC, shouldn’t be there at all. Don’t add her flaws to make her believable. Make her real. Humans are not the definition of perfection, make her human. If in this world you can conceive a human with no flaws in your story shouldn’t happen either.
Here are some tags to help you with this
Good luck (: