Kurt was a very wealthy businessman, with a very shady past.
The Doctor was there fomenting rebellion against the humans, and then promptly switched his focus to the Sontarans upon their invasion. Kurt was a smuggler who ended up in a cell with "the infamous rebel general Smith". (PROSE: Shakedown)
Kurt was part of Lisa Deranne's Tiger Moth crew. He and Deranne were the only two to survive when Sontarans boarded their solar-yacht searching for a Rutan agent, and he resolved to hire a new crew and re-enter the solar-yacht races. Prior to this event, he claimed to have met a "dentist" who had a perchant for screwdrivers and who told him about Sontarans and their weakness. He was shot by Steg after all the Sontarans on board except him had been killed, mostly by the Rutan agent. (HOMEVID: Shakedown: Return of the Sontarans)