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". The Doctor teleports to the Sontaran ship to suicide bomb the Sontarans. Luke uses the teleport to make him come back and in return takes his place on the ship and uses his invention, the Atmospheric Converter to blow up the Sontaran ship, killing himself and the Sontarans. This action saved the Earth. (DW: The Poison Sky)

He is shown as part of a series of flashbacks of The Doctor of all the people who have died (DW:Journey's End)

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. Luke had claimed that war is magnificent, but it is unknown if he truly believed this or if it was merely an attempt to curry favor with the Sontarans.