- "Pretty good. I've never heard that one before."
- "I just made it up. You liked it? My favorites are the ones with the Heterodyne boys' dopey monsters—Punch and Judy."
- Agatha and Theo 
Theopholous duMedd [pronounced doo-MEDD], or simply Theo, first appears in the story as Head Boy among the Castle Wulfenbach students. He is a known Spark. He is also well-liked, apparently for good reason, as he is a gifted storyteller and a genuinely nice guy. It eventually turns out that he is Agatha's cousin, as his mother was Serpentina Mongfish, Lucrezia's sister. He comments that until this fact is revealed, he like Agatha didn't have any living relatives of which he was aware. (It is mentioned in Agatha H. and the Airship City that duMedd Senior and Serpentina died twelve years previously while fighting air pirates.)
On Castle Wulfenbach
When Agatha Clay is brought on board the airship she winds up at the bottom of the social heap in the castle's school, Theo takes her part, just as he with a social nobody named Gil. Theo may have had a thing for Zulenna Luzhakna. It is Theo who leads the student expedition into Baron Wulfenbach's labs to peek at the hive engine. Theo gets away afterwards, thanks to help from Gil, but his reputation as a storyteller is such that the Baron immediately him as the source of a tale told by the one student who is caught. This leads to Theo's being assigned grease trap cleaning duty of indefinite duration
When Theo is himself caught using that time to instead try and build an automatic grease-trap cleaner, an unwise Lackya the Baron—in a lab where the Baron is about to cut into Othar Tryggvassen's brain, an image duMedd does not appreciate.
After Mongfish scion . (In Agatha H. and the Clockwork Princess. it's explicitly mentioned that, yes, Othar didn't actually learn Theo is a Spark when they met, only hearing it when Agatha spills the beans in Zumzum.)Agatha's escape, Theo himself escapes the airship, again encounters Othar and apparently lays on him—one that features Agatha's family connection to the heroic Heterodyne Boys and what a big fan Theo is of Othar's exploits, but seems to overlook the fact that the
A couple of months later, Theo resurfaces in Mechanicsburg, arm in proverbial arm with Sleipnir O'Hara, where they are collecting materials for a planned expedition to India to find his father's secret lab. Their lingering distrust of Gil is resolved after they overhear him declare his intentions to help Agatha. Using the Heterodyne Device, Theo and Sleipnir rescue Gil from Captain Vole's attempt to nab him.. With Zeetha, they accompany Gil into Castle Heterodyne and form part of his "pirate crew" for a short time, but wandering the castle .
He assists in cleansing Agatha, Gil and Tarvek of their Hogfarb's Resplendent Immolation, and in process is briefly by his aunt in Agatha's body, who announces that she was never very fond of her sister. Uh oh. Fortunately, Violetta is on hand to extract him from immediate danger.
After Theo is shot in the shoulder by Captain Vole, Gil takes him to recuperate at Mamma Gkika's. Sleipner of course comes along, and is rather when she realizes that the establishment is not "just a beer house." Before things can progress any further, they are both knocked out by the ringing of the Doom Bell. They are presumably still in Mamma's when the Baron activates the Take-Five Bomb and time-freezes Mechanicsburg.
Professor Kaja Foglio has said that Theo's name was a joke based on his initial fate as a casualty during the exodus from Castle Wulfenbach, but that they grew so fond of him that his life was spared. Apparently some one is now watching out for him, or Othar might have shot the young Spark back in .