Terrence Malson

Terrence Malson was a billionaire who transferred his mind into a small child named Jeff to prolong his life.

Biography
At the age of sixteen, Malson would be able to control Jeff's body. However the Eighth Doctor kept erasing Jeff's memory to prevent Malson from taking control. Jeff eventually shot himself in the head to prevent Malson from taking him over. Malson disappeared soon afterwards. (ST: Running Out of Time)