Before Popo is born in The Slave-girl from Jerusalem along with his twin brother, Soto, his mother has a dream in which she is told that her sons would save many people. Miriam dies in childbirth and Popo's father disappears soon afterwards, nearly mad with grief. For several months he is believed to be drowned. However, Gaius is found in The Beggar of Volubilis by Flavia and her friends, drunk and gambling in Roman Africa.
In the twin's parents' absence, Susannah, their grandmother and Hephzibah, Miriam's closest friend, look after them.
In The Prophet from Ephesus, Popo goes missing around the same time as several other children, leading the four detectives to thinking that he was kidnapped by Magnus. They go on a hunt for the kidnappers, but do not find Popo. The only information they discover is that he was not taken by Magnus, but by his wet-nurse, Lydia. By the end, Popo is still missing.
- Philadelphus is a male given name derived from the Greek word philadelphos, meaning 'brotherly'.
- It is very likely that Popo is still alive, as author Caroline Lawrence had started writing a new Young Adult series about him and his brother. The two grow up in completely different worlds: Popo in Britannica, Soto in the Roman Empire. However, Lawrence has stated that the writing of the series is on hold.