Xavier Simmons was a 20th century American soldier who was replaced by a Xhinn.
In December 1946, he killed Ernie Evans during a deal to buy cigarettes, which were extremely hard to get in England after World War II. He then "saw the light" and repented his actions.
By 1952, he was a priest at St Luke's Church in London. He discovered, though, that the real Xavier Simmons had been killed six years before and a Xhinn taken his place. He was paving the way for their invasion without knowing his true nature. When he refused to serve any longer, he was stripped of his Xhinn life essence, becoming fully human. He survived the destruction of the other Xhinn by the Third Doctor, and waited to see if the rest of the Xhinn fleet would arrive in 2002. (PROSE: Amorality Tale)