Gaius Suetonius Tranquillus is briefly Flavia Gemina's betrothed.
In The Sirens of Surrentum, at Pulchra's house, Tranquillus meets Flavia, whom he is betrothed to. At first he does not like her, thinking her improper and unruly. However, they begin to bond over their love for mystery and literature. Just as the two start to believe that their marriage could be a positive thing, Tranquillus' father cancels the betrothal, as he had heard reports that Flavia was not the virtuous maiden he had thought. Tranquillus is furious, knowing that his father is mistaken, but is unable to do anything to prevent him and has to leave for home.
He also appears again in The Man from Pomegranate Street. He tells Flavia that he still love her and that they should be together again. When he kisses her, she kisses him back, but it becomes clear that she now loves Flaccus.
Tranquillus is an intelligent boy who likes to laugh, though he can be snobbish and arrogant.
He is shown to, at times, be impudent, such as when he tries to persuade Lupus and Jonathan ben Mordecai to take a boat out and spy on naked women (Flavia, Nubia, Pulchra and the other female house guests in The Sirens of Surrentum) as it is "suspicious". Lupus agrees quickly, but Jonathan refuses, realising Tranquillus' true motives.