“But from that moment on, Hermione Granger became their friend. Because there are some things you can’t go through in life and not become friends, and knocking out a twelve-foot mountain troll is one of them.”
— J.K. Rowling
I love that friendship is such an important theme in the series. This passage not only describes the beginning of Ron and Harry’s friendship with Hermione but can be applied to many of the other relationships in the series. Characters become friends through their experiences and through working towards shared goals (often, but not always, the defeat of Voldemort). There is something special about the friendship of the initial three (or four if you count Neville, which I do), because their friendship journey started before they knew that they were fighting Voldemort. It started while they were still just kids starting school.