Demetrius was the slave of Marcus Licinius Crassus. He was a scribe.
At the time of Spartacus' slave rebellion, Demetrius was almost blind, though he was only 23 years old. He was afraid of what would happen when Crassus found out.
Crassus loaned Demetrius to the Second Doctor, who treated Demetrius well, much to Demetrius' confusion and discomfort. When the Doctor and his companions left the rebellion, they saved Demetrius. (PROSE: The Slave War)