Well, this takes me back to some of my early attempts at writing for the 'Harry Potter' fandom. I thought I was being so clever, creating this brilliant, gorgeous witch who could out-duel Voldemort and had a tragic past. Looking back, she was textbook. The biggest lesson I learned? Give your character a real, tangible flaw that isn't just 'she’s too kind' or 'cares too much'. Make it something that genuinely hinders their goals and forces them to adapt. A sharp tongue that alienates allies, a physical limitation they have to work around, or a blind spot in their judgment that causes real setbacks.
Another pitfall is having the established cast revolve entirely around the new character for no clear reason. Why would Hermione instantly be best friends with this random transfer student? Build that relationship with the same slow-burn care you'd use for a canon relationship. Let there be initial distrust, clashes of personality, or simple indifference that has to be overcome through shared events. The world shouldn't bend to make them the center; they have to earn their place in it, with all the friction that entails.