Theo was a member of the terrorist group lead by Hans Gruber in Die Hard. He was well-aware of what the group was doing and didn't seem to care much. He even participated in full with it. He was depicted as weasely, insensitive and arrogant, cracking jokes whenever people were killed. Theo and Eddie were the only American terrorists in Hans' group. He is the tertiary antagonist in this film.
He was responsible for breaking the vault open and rob the 640 million $, while the others would ensure, that they would get away with it and then flee together in the van prepared to escape with the money undetected.
He was spotted and knocked out by Argyle and it is assumed, he was arrested by the police later.