It could be thought that she had family in Istria, now in Croatia, based on a trip she took there (Suet. Dom. 12.3). In her 30s Caenis, still possibly a slave, was in an unofficial type of relationship with Vespasian, known as contubernium, before his marriage. According to Suetonius, after the death of Vespasian's wife Flavia Domitilla, Vespasian and Caenis, now a freedwoman, resumed their relationship; she was his wife "in all but name" until her death in AD 74.
She had a remarkable memory and considerable influence on the emperor's administration, carried out official business on his behalf, and apparently made a lot of money from her position. However, she was treated with disrespect by Vespasian's son Domitian, who refused to greet her as one of the family.
The life of Caenis and her love-story with Vespasian is portrayed in Lindsey Davis' novel The Course of Honour. She is also a character who features regularly in Robert Fabbri's Vespasian series, where she is depicted as being the long lost grand-niece of the king of the Caenii, a rebelling tribe in Thracia.