Jackle

Jackle  is the Sneakery teacher at Hieronymous Wiggenstaff's School for Heroism and Villainy. His classroom is the only one with a balcony in the school. He is a member of the Unbroken Chain along with Argo Keene's mother, Shebrie Keene. He is the one to invite Argo to the Unbroken Chain.

Argo Keene
Jackle and Argo have formed a sort of mentor-mentee bond, with Jackal asking Argo to steal things for him. Jackle remains Argo's primary contact with the Unbroken Chain.

Shebrie Keene
Jackle and Shebrie were close during their formative years. They attended school and sailed together. Both were members of the Unbroken Chain, and Shebrie entrusted Jackle to give her invitation to Argo before her death.