Luke Rattigan

Luke Rattigan was a child genius

Luke was the founder of the Rattigan Academy. Being just 12 year old, he had invented the Fountain 6 search engine, becoming a millionaire overnight so. As a frustrated genius who was belittled by the society and still as a teenager, he cooperated with the Sontarans and invented the ATMOS car device (DW: The Sontaran Stratagem), which would poison the Earth atmosphere so the aliens would build a clone army to fight their own war at their planet.

The Doctor arrives when the now 18 year old Rattigan was directing his own Rattigan Academy which was formed by other gifted teenagers of similar profile. For all them, the Sontaran had promised the nonexistent Castor 6 planet.

However when finally Luke declares war he is magnificent and tries to convince the pupils of the Rattigan academy to come with him to that planet to found their own society, for which they had been developing things in their labs. 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 merely 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

 * "No one's said 'no' to you in a long time, have they?"
 * --The Doctor

Luke's incredible intelligence and early financial success gave him an inflated sense of self-esteem, to the level of megalomania. Like a child star, he likely grew up with a sense that there was nothing he could not, or was not allowed to do. His disdain for those not as clever as he is clear in his behavior. 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 favour with the Sontarans.