Luke Rattigan
Luke Rattigan was a gifted teenager who developed the Atmos car technology in cooperation with the invading Sontaran species. Also, Rattigan was the founder of the Rattigan Academy.
Biography
Despite being just 12 years old, Luke Rattigan invented the "Fountain 6" search engine, becoming a millionaire overnight by this reason. Then, feeling misanthropic and underappreciated by society, he cooperated with the Sontarans to develop the "Atmos" car device, which effectively eliminated all CO2 emissions of car but which would poison the Earth atmosphere in time for the Sontarans' stratagem. Then, the aliens would build a clone army to fight their own war against the Rutan Host.
The Doctor arrived when the then 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 36" planet, which would be renamed "Earth-Point-Two" by Rattigan.
Eventually, the Sontarans activated their "Atmos" devices, which had already been installed into the 50% of all world automobiles. They emitted a gas which would incapacitate all humans for their cloning process and such cloud was soon wrapping the whole planet. Declaring war, Luke got a magnificent demeanor and tried to convince his pupils of his academy to come with him to "Castor 36" and to found their own civilization (for which they had been developing things in their labs). However, they abandoned Rattigan then, thinking he was mad.
Luke returned to the Sontaran mothership and told the Sontarans of his failure. The Sontarans revealed they had no plans to keep them alive anyway and merely had needed Luke and his pupils, at that point, "for installation of the Atmos system." Luke teleported again back to Earth and he was met again by the Doctor, who told him to "do something clever with his life". The Doctor teleported again to the Sontaran ship to suicide bomb the Sontarans but Luke used the teleportation device timely to make him come back and in return Luke took his place on the ship and used his invention, the Atmospheric Converter to blow up the Sontaran ship, killing himself and the Sontarans. This action saved the Earth.
When Davros asked the doctor "How many have died in your name" Rattigan appeared in a flashback of people that have died. (DW: Journey's End)
Personality
Even though Luke was incredibly brilliant, he had several mental problems. Luke found the Doctor annoying because of his cheeky attiude and intelligence. It seemed that the reason for Luke's problems was because of lack of social skills, which was shown when he believed all his students had been rejected from society for being intelligent.
Luke found murder and suffering of Humans entertaining because of his hatred towards the world. Luke appeared to have joined forces with the Sontarans because they appreciated his genius leading him to commit global treason.
