Who is Zach's Mother from Only the Good Spy Young and what is their importance?

Late in the novel, Zach and Cammie are on a mission in the secret caves beneath the Blackthorne Institute, where they encounter a group of Circle agents torturing Joe Solomon for information. The lead Circle agent is identified as Zach's mother, which explains Zach has never been harmed. Zach's mother is described as ruthless.