We meet Katie, who is described as Robin’s favorite cousin, at Robin and Matthew’s wedding reception. She is heavily pregnant as their wedding falls on Katie’s due date. She is the one to tell Robin that Strike is at the wedding reception: “’He came, then,’ said Robin’s favorite cousin, Katie.’” 1 

In Troubled Blood, Katie makes a statement that not only upsets Robin, but also sticks with her for quite some time: 

“‘It’s like you’re traveling in a different direction to the rest of us.’”

While these words initially bother Robin, she is finally able to find peace with them when she realizes what it really means for her. 

“But listening to Court and Spark, Robin thought that it was perfectly true that she was traveling in a different direction to anyone she knew. She was fighting her way back to the person she should have been before a man in a mask reached for her from the darkness beneath a stairwell. The reason nobody else understood was that they assumed that her true self was to be the wife Matthew Cunliffe had wanted: a woman who worked quietly in HR and stayed home safely after dark. They didn’t realize that that woman had been the result of those twenty minutes, and that the authentic Robin might never have emerged if she hadn’t been sent, by mistake, to a shabby office in Denmark Street.” 3

In The Ink Black Heart, Robin tells Strike that Katie is trying to set her up with a friend of hers who has just moved to London, who goes by the name of Axeman (Ch 1). Robin accepts an invitation from Katie to go skiing over New Year’s in Switzerland with Katie’s husband and son and some friends, including Hugh ‘Axeman’ Jacks (Ch 2).

1: Lethal White, Prologue
2: Troubled Blood, Chapter 9
3: Troubled Blood, Chapter 30