Castor was a djinni in the service of Nathaniel for a short time.
Mentioned in book 2, Castor was last summoned during the Italian Renaissance.
He was courteous but proud. (also still in the same page of book 2) He caused a rumble between the foliots of other magicians in Nathaniel's work-place. Nathaniel quickly dismissed the djinni to evade any investigation on who started the fight. Nathaniel never summoned Castor again to avoid suspicions.