Strike’s brother-in-law, Greg, is a quantity surveyor and lives with Lucy and their three sons in Bromley.1 In The Cuckoo’s Calling, Strike sees Greg at his nephew Jack’s birthday party where Greg “manned the iPod.” Strike isn’t entirely fond of Greg and finds his tone of “chippiness and aggression” to be “irksome.”2 

It probably doesn’t help with Strike’s opinion of Matthew that Lucy later compares him to Greg when she first meets Robin. “Oh, her fiancé — he sounds a bit like Greg.”3

Greg doesn’t do much better for himself in The Silkworm when Strike reluctantly attends his own birthday party at Greg and Lucy’s house. Greg gives Strike some unsolicited business advice and makes an uncomfortable comment to Nick and Ilsa.4

“‘And still no kids, all these years later?’ asked Greg, smug father of three sons.

There was the tiniest pause. Strike knew that Nick and Ilsa had been trying for a child, without success, for several years.” 5

Strike speaks to Greg over the phone in Lethal White when Strike is with Jack in the hospital. Greg takes the phone from a distraught Lucy and it’s “… the first time in Strike’s life, he was glad to hear Greg.” 6

This feeling doesn’t last for long when later Greg lectures Strike about his business, yet again.

“He generally avoided discussing his business with Greg, who seemed to have been wrong-footed by Strike’s success. His brother-in-law had always given the impression of being irritated by Strike’s distinguished military career. As he fielded Greg’s questions about the structure of the business, the rights and responsibilities of his freelance hires, Robin’s special status as salaried partner and the potential for expansion, Strike detected, not for the first time, Greg’s barely disguised hope that there might be something Strike had forgotten or overlooked, too much the soldier to easily navigate the civilian business world.”7

Greg is occasionally around in Troubled Blood while the family gathers in St Mawes to be with Joan during her illness. Lucy mentions a business trip to Wales8 and he is later seen on a business call when Jack goes with Strike and Polworth to the pub.9 

1,2: The Cuckoo’s Calling, Part 3 Chapter 2
3: The Cuckoo’s Calling, Part 3 Chapter 9
4, 5: The Silkworm, Chapter 15
6: Lethal White, Chapter 26
7: Lethal White, Chapter 37
8: Troubled Blood, Chapter 39
9: Troubled Blood, Chapter 44