Roche was a Time Lord, a member of the Arcalian Chapter. He had been attempting to save Caresh from a passing neutron star that would have rendered the planet uninhabitable by forcing Caresh into a permanent orbit around its warmer sun, Beacon, but was interrupted by the Furies, who worried that his actions would damage the pocket dimension where they lived, forcing him to flee to Earth.
He could swap mindscents with humans, effectively adopting their appearances for a short time, during which period they resembled him. Roche's TARDIS disguised itself as a shower cubicle and an ice cream van before he disposed of it as part of a plan to trap the Furies.
After spending some time in a self-induced coma to try and throw the Furies off, he was interrupted during an escape attempt when he was hit by a bus, causing him to regenerate, although he was able to control the process enough to ensure that he regenerated into a body identical to the Doctor's third incarnation.
The Doctor was later able to avert the events that prompted the Furies' masters to send them after Roche — realising that Roche didn't need to send Caresh into a permanent orbit around Beacon but just nudge it into a closer orbit around its cooler star, Ember — restoring his TARDIS and allowing him to leave Earth. (PROSE: The Suns of Caresh)