Dr. Brimstone is one of the most brillant teachers at Nathan's school in Rebel City, but he's actually one of Professor Wilder's assistant in the Monster Empire.
Warrior's Awakening Arc
Nathan discovered at a Monster Empire hideout that Dr. Brimstone was working with the Monster Empire by kidnapping Emily and some other children to use as test subjects for experiments. He was being observed by Aviva to find out what is making some progress so slow. When Nathan saw what Dr. Brimstone was doing, the latter sent some bat monsters to capture him for seeing too much, but an accidental energy blast from Nathan was shot at a power generator causing the whole place to self-destruct. When the authorities arrived to save the children; Aviva managed to escape, but Dr. Brimstone was arrested and taken to the United Military Prison.