Quintus was more willing than the rest of his family to help the Tenth Doctor. Against his parents' express wishes, Quintus guided the Doctor to the house of cult leader Lucius Petrus Dextrus. Later, at the Doctor's instruction, he killed a Pyrovile that appeared in his family's house by dousing it in water.
Though he seemed to be a jaded and rebellious youth, performing his religious duties to the family's household gods only under duress, Quintus was secretly concerned for his sister's health. Evelina was suffering from a mysterious ailment due to her inhalation of Pyrovile dust, as well as experiencing prophetic episodes.
Quintus, along with the rest of his family, was saved from the eruption of Mount Vesuvius by the Doctor. Six months later, the Caecilius family was living in Rome and Quintus was studying to become a doctor. He also followed the traditions/religion of his family more willingly, as the Caecilius family's new household gods were the Tenth Doctor and Donna, and the TARDIS was seen as a temple. (TV: The Fires of Pompeii)