Thermus was a short man with a long scar running across his face. He worked with Flaccus in the Colosseum dungeons. He didn't care for justice at all, and laughed at the Tenth Doctor's claims that what they were doing was unlawful. Both he and Flaccus made snarky comments, believing themselves to be the height of wit.
He tricked the Doctor into thinking he was going to be released, saying he must have friends in high places, when he was actually going to chuck him into the arena to be hunted by animals. He didn't seem to care that the Doctor grabbed his sonic screwdriver on the way there.
During the revolt and escape of the prisoners, he was sent out with other guards to try to put the riot down. Both he and Flaccus stayed back whilst the others were slaughtered by the sheer number and rage of the prisoners. Attempting to back out of the arena when the group turned to them, he tripped over a lion that the Doctor had previously put to sleep. Consequently, the lion woke up and killed him. (PROSE: The Stone Rose)