Jonathan is the youngest of Robin’s brothers. We first see him in The Silkworm as he returns from the corner shop with milk, requested by his mother.1
“‘It’s on the front page, Rob. That writer with his guts ripped out—'” Jonathan tells Robin, shoving a copy of the Daily Express under her nose for her to read.2
In The Silkworm, on the day of a certain funeral, Jonathan and Robin’s other two brothers, Stephen and Martin, creep out of the house to go to The Bay Horse pub to see their mates, leaving a miserable Matthew behind without invitation.3
In Troubled Blood, Jonathan and two of his friends, Courtney and Kyle, stay with Robin over Valentine’s Day weekend. Robin is already hesitant to have a couple of strangers stay with her, but the weekend further dissolves into chaos as Strike clashes with Jonathan’s friends.4
1, 2: The Silkworm, Chapter 32
3: The Silkworm, Chapter 34
4: Troubled Blood, Chapter 40