Luke Rattigan

Luke Rattigan was a child genius

Luke was thefounder of the Rattigan Academy. At first he was just a clever person but then invented a remarkable engine and became a millionaire overnight. He was just twelve. He cooperated with the Sontarans and invented the ATMOS. (DW: The Sontaran Stratagem). Luke declares war is magnificent and tries to convince the pupils of the Rattigan academy to come with him to a new planet. They leave him thinking he's mad. Luke returns to the Sontaran ship and tells the Sontarans of his failure. The Sontarans reveal they had no plans to keep them alive anyway and mearly needed Luke and his pupils to create the ATMOS. Luke teleports again back to Earth and eventually is met by the Doctor. The Doctor tells Luke to "do something clever with his life". When the Doctor teleports to the Sontaran ship to suicide bomb them Luke uses the teleport to make him come back and in return takes his place and blows up the Sontaran ship, killing himself. (DW: The Poison Sky)

Personality
Luke was quite clearly a sociopath, with delusions of grandeur given his decision to name the new world after himself. It is debatable whether his choice to kill the Sontaran's was a heroic decision or one born of spite over having been betrayed by his alien allies