This is something I thought of but as they meet at school, there should be a flashback of when they first met and became friends. I was thinking that since he's different, he was avoided by others at school, but when y/n came along she didn't avoid or judge him and thought of him as a amazing person to hang out with, which she becomes his first friend. Thinking about this shows how they formed a special bond by treating each other as family that it will last forever.
Also, should they have a nickname for each other? For example, y/n calls Mephiles Abyss because of his powers, and he calls her Hotshot because she is strong and confident. Do you think they would call each other those names, or would you change them?
Sorry if this is too much. I just have so many ideas for your book.