Theodore was born in Esterholt to a mother who was a fairly powerful mage of some sort and his father was not proud when Theo started to show an affinity for the magical arts. As he learned from his mother, his father disapproved more and more so Theo put his skills on the shelf and became something of a minor level thug.
One night while making some coin in a bar called Safe Haven in Brightport, he was approached by Yenward, Duncan and Malchus as they entered and they invited him to drink with them. Against his will and some added peer pressure he got drunk and passed out and was carried back to House Dragonkeep to sleep it off.
The next morning, his head pounding and his desire to die rising, he is mercifully rescued by Tessie Grimnas and something of a hangover cure to relief the pain.
Afterwards, he joins the party as they leave Brightport on their way to Bandit's Rest where he helped kill a large red dragon with the group.
During a dragon attack where Derthag is killed, Theo launches a lightning bolt, much to the surprise of everyone.
it is revealed that his mother is quite a powerful mage in Esterholt and apparently he has some aptitude towards the arcane.
Theo continued to travel with Group 1 and under the tutelage of Malchus and became a reasonable adventurer. While fighting some bandits however, his throat was cut and he narrowly survived due to quick thinking from Teveen.
Sometime after Episode 200, with Aria losing her powers, she and Theo married and settled down in Winterhaven. With Malchus, they became emissaries of the city.
Theo is about 5'6" with short cropped hair and a beard. He wears leather armor and fancies himself to be something of a thug. As it turns out, Theo is a softy who would have been a sad example for aspiring assassins.