Agathus is an elderly, Jewish door slave to Titus' palace.
During The Assassins of Rome, he befriended Jonathan ben Mordecai and his uncle, Simeon ben Jonah. When Jonathan was captured and enslaved, Agathus was kind towards him and helped him deal with the pain of the branding. It is apparent that he was either an ex-freedom fighter or an ex-assassin, as he takes Simeon's sica as some form of password. He despises the Romans, and it is implied that he was there at the Sack of Jerusalem.
He appeared once more in The Enemies of Jupiter, where it became clear that he was the agent of potential assassin, Berenice, who was determined to kill Susannah bat Jonah, sister to Simeon and mother to Jonathan. Agathus led Jonathan into believing that his mission was to kill Titus, although Jonathan eventually guessed his true plan: to burn down Rome. They engaged in a fatal battle, in which Agathus, despite being armed with a knife, was knocked into the flames by the terrified boy and perished.
Agathus has appeared in the following novels: