The Circle of Cavan is an ancient group founded by Ioseph Cavan, who attempted to kill Lincoln but was thwarted when Gillian Gallagher stole his sword and killed him: the sword that is on display at the Gallagher Academy. Cammie realized who they were when she saw that one of the members wore a ring that had a symbol that also appeared on Gilly's sword. They intend to try and kidnap Cammie for reasons unknown. The Circle of Cavan is introduced in DJGC, when they give Macey misleading instructions and lock her on the rooftop with Cammie and Preston, the potential president's son, so they could kidnap Cammie. In most of DJGC, it is believed Macey/Preston was the object of the kidnapper's interest. Nobody really knows why they are trying to kidnap Cammie.
Known members of the Circle include Zach's mom, Catherine Goode, a woman with dark red hair and dark brown eyes like Zach's and Dr. Steve. Most, if not all, of the Circle of Cavan members are double agents, and agents are often recruited young while at spy schools, including the Gallagher Academy and the Blackthorne Institute, like they did Joseph Solomon when he was 16. Although, considering Zach's mom was from Gallagher, it is possible they also recruit some from there as well. The only people to have ever gotten close to stopping the Cavan are Joseph Solomon and Matthew Morgan (Cammie's dad). The Circle is believed to have killed Matthew Morgan, although in Only the Good Spy Young, Zach's mother alleged that he was alive.
In book five it is revealed that the Circle is after Cammie because they want her to remember the night when she went to the circus with her father. She saw a list of the list of founding members of the Circle of Cavan. This was the same information that the Circle tried to get out of Matthew Morgan all those years ago, eventually leading to his death, evidenced by Cammie being shown his grave after she remembered where the Circle had held her captive the summer before.
Members of the Circle of Cavan include:
Ioseph Cavan (founder)
Original members of the Circle:
Thomas Avery McKnight
Samuel P. Winters
Descendants (who are current members of the Circle):
Sir Walter Knight
Samuel Winters (Preston Winter's father)
Current members of the Circle:
Dr. Steve Sanders
Maxwell Edwards (also known as Agent Edwards)
Former members of the Circle: