“The bloke who told me who’d stabbed that police informer, remember?” 1

This is how Strike first describes Shanker to Robin. Shanker is all this and more.

The first time Robin sees Shanker, she’s alone at the office, so his appearance terrifies her. He is described as “gaunt and pale, his head was shaven, a few freckles were scattered across a broad nose and his mouth was wide and thick lipped. Tattoos covered his wrists, knuckles and neck…. A deep scar ran from the middle of his upper lip towards his cheekbone, dragging his mouth upwards in a permanent Elvis-style sneer.” 2

But Strike later assures Robin that Shanker would never hurt her. He might rob her blind, but he wouldn’t harm her.3

Shanker and Strike (who Shanker calls Bunsen, for reasons Strike no longer remembers4) met when they were teenagers5. Strike’s mother, Leda, took Shanker in at their Whitechapel squat after he was hurt in a gang street fight, and she looked after him (as best she could) while he healed. He ended up staying around, becoming fiercely loyal to Leda and somewhat of a brother to Strike.6

As adults, Shanker and Strike are old friends, but mainly business associates of sorts. Shanker is Strike’s connection to London’s criminal underworld, providing him useful information and intel — such as the whereabouts of Jeff Whittaker.7 But “Strike never forgot what Shanker was, nor did Shanker ever forget what Strike was,” 8 as they work on opposite sides of the law. And it’s always business — Strike pays Shanker for every service performed. As Strike says to Robin, Shanker “does nothing for nothing.” 9

Robin keeps this in mind for later….10

In Troubled Blood, Strike meets up with Shanker at the Shakespeare’s Head to ask for information on one of the key suspects in Margot Bamborough’s disappearance, Niccolo Ricci. This is the first time Shanker ever warns Strike away from someone and makes it clear how dangerous the Ricci family is.11 

Shanker is apparently still dating Alyssa from Career of Evil, as he mentions that he’s Christmas shopping for Alyssa’s daughter Zahara and that she’s recently gotten over the flu.12 

“‘Well, keep it the fuck away from me,’ said Shanker. ‘Last fing we fuckin’ need at home. Zahara’s only just got over the fuckin’ flu.’” 13

In an interview for Troubled Blood with Mark Billigham, J.K. Rowling said she’d created the character of Shanker in 2010, way before his appearance in Career of Evil in 2015.

1: Career of Evil, Chapter 8
2: Career of Evil, Chapter 18
3: Career of Evil, Chapter 20
4, 5: Career of Evil, Chapter 10
6: Career of Evil, Chapter 18
7: Career of Evil, Chapter 33
8: Career of Evil, Chapter 10
9: Career of Evil, Chapter 20
10: Career of Evil, Chapters 53 & 54 
11-13: Troubled Blood, Chapter 27