Husband of Metella and father to Evelina and Quintus, Caecilius was a marble trader with political ambitions; it was to impress the augur Lucius Petrus Dextrus, that he bought the Tenth Doctor's TARDIS at the market, intending to display it as "modern art".
While initially planning to leave them to suffer the horrors that history had planned for them, the Tenth Doctor was convinced by Donna Noble to intervene and save Caecilius along with his family. As history would not record their survival, the Doctor thus did technically not change history. By early 80, Caecilius and his family had resettled in Rome, and adopted the Tenth Doctor and Donna as their household gods. At this time, Caecilius was finalizing a major business deal with the Egyptians, which would ensure his family's financial security for the rest of their lives. (TV: The Fires of Pompeii)
The Doctor later took on the face of Caecilius when he regenerated into his twelfth incarnation. (TV: The Time of the Doctor) Although unsure why his new face appeared familiar at first (TV: Deep Breath), he came to realise that it was to remind himself that his purpose as the Doctor was to always save people — no matter how impossible or "wrong" it might seem. (TV: The Girl Who Died)
Behind the scenes
Caecilius is notable for being one of the three characters played by Peter Capaldi within televised Doctor Who media, along with the Twelfth Doctor and John Frobisher. In the Torchwood Declassified episode Cracking Children of Earth, Russell T Davies suggested that, since they are played by the same actor, John Frobisher may be a descendant of Lobus.
The name game
This character's surname was — by Russell T Davies' admission in The Italian Job — derived from books in the Cambridge Latin Course, elementary Latin books about real, historical people. For this reason, it is frequently asserted that this character is meant to be a "celebrity historical character" in the vein of Charles Dickens or Agatha Christie. Many people, including at least one official merchandise licensee, believe that this character is supposed to be the historical Lucius Caecilius Iucundus.
Though this theory is both plausible and beguiling, the script actually names him Lobus Caecilius; the words "Lucius" or "Iucundus" are never used in connection with Capaldi's character. In The Italian Job, the production team specifies that they merely took the names from Cambridge Latin Course books: as Phil Collinson confirmed, "It's all in [the Cambridge Latin Course] — in terms of the names". But it was not the authorial intent that this character or his family be an attempt at recreation of historical figures. Instead, Davies characterised the use of the name "Caecilius" as a gag, supporting the many other broad Latin gags in the episode, similar to those found in his beloved Asterix comics.