Jeremias Tetch

Jeremias Tetch is a mentally deranged and dangerous terrorist, criminal and murderer. He is also the more intelligent and more dangerous identical twin brother of the anarchistic cult leader Koschei Tetch. After his brother's death, Jeremias takes over the role of main antagonist for the final arc of Series 6.

As a child, Jeremias supposedly suffered years of abuse at the hands of his psychopathic brother, who even burnt Jeremias' bed while Jeremias himself was in it. Jeremias would in turn manufacture several lies about his brother that would turn the reincarnated Koschei's entire family against him; it remains unclear how much truth there was in either story. In a desperate attempt to defend Jeremias from his monstrous brother, his uncle took him away from him and proceeded to take him to Southern Gallifrey. Following this Jeremias changed his name and started calling himself Nazo Viter. After finishing school, Jeremias became a brilliant engineer, designing highly advanced generators for Asgard Electronics. He also built a labyrinth hideout to protect himself from his brother, which took almost six years of construction. In addition to the hideout which he filled with security cameras, rooms of work and more, he met a young woman named Shiori who started to work for him as a proxy and assistant. Jeremias would later reunite with his brother, who after a rampage to find his twin, found him in his hideout.

Right after witnessing his brother's death, Jeremias received a gift sent by The Master, which is revealed to be a "special" gas created just for him. Upon inhaling it, his face became pale white and a wide red grin formed from ear to ear, finally giving Jeremias the drive to explore that dark side of him that his brother had seen. Becoming obsessed with both Sentineli Sigma (whom he viewed as a kindred spirit) and re-building Gallifrey in his own image after destroying it. After shooting Kotori Sigma in an attempt to destroy Commander's attachments and drive him insane, Jeremias killed the president and demolished all of Arcadium's bridges, cutting the city off from the rest of the world. With Arcadium reduced to a lawless  "no-man's land", Jeremias managed to escape into the night, remaining on the loose in the abandoned city.