His brother James
In a second, undisputed passage, Josephus records the illegal execution of James in AD 62, and identifies him in passing as “the brother of Jesus who was called Christ.” That he mentions Jesus without explanation suggests his readers already knew whom he meant.
When therefore Ananus was of this disposition, he thought he had now a proper opportunity [to exercise his authority]. Festus was now dead; and Albinus was but upon the road. So he assembled the sanhedrim of judges, and brought before them the brother of Jesus who was called Christ, whose name was James: and some others; [or, some of his companions.] And when he had formed an accusation against them as breakers of the law, he delivered them to be stoned.