Dev Ashton was an engineer on-board the SS Pentallian. Erina Lessak worked under him.
The sentient star Torajii, having already possessed another crewmember, took over Ashton's body to make him kill. Due to the intervention of the Tenth Doctor, Torajii relinquished its hold over Ashton. After Ashton had been possessed, Kath McDonnell tricked the possessed Ashton into being trapped in the stasis chamber, which killed it. (TV: 42)