Mordecai first appears in The Thieves of Ostia when he bandages Flavia's leg and the family become great friends of the Geminus family. When his son Jonathan meets a homeless boy named Lupus, he takes him and he becomes a member of the family.
In the second book, The Secrets of Vesuvius, Mordecai goes to stay with his children at Flavia's uncle Gaius' farm in Pompeii, but soon wants to leave, as the area gets frequently disturbed by tremors. However, he refuses to depart without Miriam who has fallen in love with their host. He then proves to be calm in the face of crisis, when his fears are realised and Mount Vesuvius erupts.
At the start of The Pirates of Pompeii, we discover that he hates the Emperor Titus for the attack on Jerusalem that supposedly resulted in his wife's death. However, in The Assassins of Rome, after meeting his wife's brother, Simeon ben Jonah, his son runs away and discovers that she is not dead, but is forced not to tell his family because she is a slave in the Emperor's Golden House. During this, Mordecai is in jail for being accused of helping Simeon. At the end of the book Mordecai and his family are given Roman citizenship.
When Flavia's father, Captain Marcus, arrives at the house injured and bankrupt at the start of The Dolphins of Laurentum, Mordecai helps him to recover, and when Pliny offers them a stay in his villa, he accompanies them to help look after Marcus. When the detectives see that the notorious slave-dealer Venalicius is trying to get the treasure they are attempting to reach, they think he is trying to stop them. Mordecai tells them that the man is trying to help, but they refuse to believe him. By the time they are persuaded, Venalicius is on his deathbed, with Mordecai tending to him. We discover that, while Mordecai was a prisoner in The Assassins of Rome, Venalicius had repented and wrote him a confession of all the things he had done to Lupus and his family, which he reads out.
Mordecai plays a significant role in The Enemies of Jupiter. At the beginning, Jonathan suspects he is courting one of his Roman patients as he is trying to look Roman; Jonathan knows that his wife Susannah is really alive and decides he needs to show his father. Mordecai and the detectives are summoned to Rome, supposedly by the Emperor, to tend to the victims of the plague sweeping through the country. Jonathan takes advantage of the fact that this is very nearby the Golden house, where Susannah is a slave and tries to bring them together. Titus appears and is furious, he orders his guards to arrest Mordecai. Suddenly, Susannah falls ill and the detectives realise she has been poisoned. They manage to find the andidote, but soon after they have saved her a terrible fire breaks out in the city. When it has died down, Jonathan is believed to be dead. Susannah comes home with Mordecai, but they pretend she is dead to stop further assassination attempts by Berenice.
However, Jonathan is alive and fighting as a gladiator in Rome. He is consumed by guilt, as he believes that he started the fire and that his mother is dead. Mordecai stays at home with his wife during this book, The Gladiators from Capua, but Jonathan's arrival home at the end must have affected him.
The next book of the series that Mordecai appears in is The Slave-girl from Jerusalem. He seems to be more cautious and insists the four detectives need some supervision with Hephzibah's case. This is understandable, as he had lost his son for months and felt he needed to keep him safe. He is with them for most of the adventure and helps them solve the mystery of Dives' death. However, at the end of the book his daughter dies after giving birth to his grandsons, Popo and Soso.
After his daughter's death, in The Beggar of Volubilis, he has resorted to taking opium, then known as 'poppy tears'. His voice is slurred and he mutters constantly about his regrets.
Mordecai and Susannah do not appear in The Scribes from Alexandria, but it is mentioned that they again believe their son is dead. However, they later find out that he is alive and returning to Ostia.
By the time of the events in The Man from Pomegranate Street, Mordecai has become consumed with a hatred of Emperor Titus and a desire for revenge. He plans to assassinate him, but changes his mind after catching a glimpse of a boy who resembles Jonathan, not realising it is indeed his son. The Emperor dies mysteriously not long after and Mordecai becomes a prime suspect for murder. In order to save him from death, Jonathan falsely admits to the assassination of Titus and he is tortured. Through the intervention of Flavia, Nubia and Lupus, their friend's name is cleared and weeks later, so is Mordecai's.
- Mordecai is a male given name, thought to be of Hebrew origin. It means 'servant of Marduk', which is sometimes interpreted to mean 'warrior'.