Strike meets Robin’s eldest brother Stephen when he’s seated with him and his wife at Robin and Matthew’s wedding reception. While initially appearing to be skeptical of Strike, Stephen warms to him quickly and appears glad that Strike wants Robin back. He tell Strike that he’s never seen Robin happier than when she was working for him. Stephen’s physique is large.¹

In Troubled Blood, Stephen, becomes a dad when his wife, Jenny, gives birth to their baby girl shortly before Christmas. 

“‘My God, Button,’ she said quietly; it was Stephen’s childhood nickname. ‘You’re a dad!’” ²

Robin spends some time with her brother and the rest of the Ellacotts when she goes home for Christmas. 

“The front door opened before the car came to a halt. Jenny and Stephen were standing on the threshold, looking so excited that an onlooker might have suspected it was they who were about to see their baby daughter for the first time, not Robin.” ³

^{1: Lethal White, Prologue
2: Troubled Blood, Chapter 23
3: Troubled Blood, Chapter 28}