We first meet young Zahara in Career of Evil when Robin goes to warn her mother about Noel Brockbank.1 Robin’s choice to ignore Strike’s warning to stay away from Brockbank and attempt to save Zahara is the thing that causes him to (briefly) fire her.2
Shanker assisted Robin with her rescue mission and became acquainted with Zahara’s mother, Alyssa.3
In Troubled Blood, it appears that Shanker is living with Alyssa when he mentions Zahara twice to Strike.4
1: Career of Evil, Chapter 54
2: Career of Evil, Chapter 55
3: Career of Evil, Chapter 54
4: Troubled Blood, Chapters 26 & 27