Agnes Waite was the mother of serial killer Dennis Creed. We learn about her in chapter 8 of Troubled Blood when Strike reads her interview in the book The Demon of Paradise Park. 

Agnes tells the interviewer about the abuse she suffered at the hands of her stepfather, William Awdry, which eventually resulted in her pregnancy. While Awdry threatened to kill Dennis when he was born, Agnes begged him not to and his life was spared. Eventually, Agnes had to leave her son behind in order to escape her abuser. 

When talking about what Creed became, Agnes said, “‘After the trial was over, I thought back to him, all naked and bloody on the lino where I’d had him, with my stepfather standing over us, threatening to drown him, and I swear to you now,’ said Agnes Waite, ‘I wish I’d let it happen.’” 

^{Troubled Blood, Chapter 8}