Prologue:
It wasn't one thing that led up to her running away. It was one unbearable month of Ron's hexes, jinxes, and rude comments; and Hermione's bossy orders, intrusive questioning, and condescending commentary; and Harry's general obliviousness to how much he taking from her.
Lyra tried to be understanding at first. Harry was Sirius's godson, and in another world, they would have probably grown up like siblings. Hermione and Harry had saved Sirius's life in their third year, and Ron had known him for just as long. Of course they would be close, and of course the Golden Trio would always be in Grimmauld Place together. But it just got harder and harder for Lyra to make excuses for the trio's actions (or inactions) towards her.