Luke Rattigan

Luke Rattigan was a gifted and ambitious teenager who developed the Atmos car technology in cooperation with the invading Sontaran race. He was the founder of the Rattigan Academy, a school for geniuses.

Biography
At twelve years old, Luke invented the "Fountain 6" search engine, which made him a millionaire almost overnight. In his later teens, feeling misanthropic and unappreciated by society, he worked with the Sontarans to develop the "Atmos" car device, which effectively eliminated all CO2 emissions of a car. It would also poison the atmosphere so the Sontarans could build a clone army to fight their war against the Rutan Host.

The Tenth Doctor arrived when the then-eighteen-year-old Rattigan was running his own Rattigan Academy, teaching similarly gifted teenagers handpicked by Rattigan. Unbeknownst to any of them but Luke, the Sontarans had promised to give them nonexistent "Castor 36" planet (renamed "Earth-Point-Two" by Rattigan) to build a new world after the poisoning of Earth.

The Sontarans activated their "Atmos" devices, which had been installed into 50% of the world's automobiles. The Atmos systems emitted a gas which would kill all humans for their cloning process. Clouds soon wrapped the whole planet. Overcome by the thrill of war, Luke teleported from the Sontaran ship where he'd been watching the gas enveloping the planet back to his academy. He tried to convince his pupils to come with him to "Castor 36" to found their own civilisation (for which they had been developing superadvanced technology in their labs). Horrified by the disaster and taking Rattigan to be mad, they abandoned him and fled.

A dejected Luke returned to the Sontaran mothership and reported his failure. The Sontarans revealed they had had no plans to keep the students alive anyway. They had needed Luke and his pupils for the creation and installation of the Atmos systems. Devastated, Luke teleported again back to Earth. He was met again by the Doctor, who told him to "do something clever with his life". The Doctor teleported to the Sontaran ship with a reversed Atmospheric Converter. He intended to blow up the Sontarans—and himself in the process—if they refused to leave Earth alone. Taking the Doctor's advice, Luke reprogrammed the teleporter, switched places with the Doctor and blew up the alien ship just in time, killing himself in the process. When Martha and Donna (who were present on Earth with the Doctor) asked him what he was doing when he reprogrammed the teleporter, he said, "Something clever." (DW: The Sontaran Stratagem/The Poison Sky)

When Davros asked the Doctor, "How many have died in your name?", Rattigan appeared in a flashback of people who died. (DW: Journey's End)

Personality
Even though Luke was incredibly brilliant, he had several idiosyncrasies, behaving neurotically, in an outburst of unreasonable emotion. Luke lacked social skills. He believed all his students had been rejected from society for being intelligent. He was very ambitious, and callous towards people he felt did not meet his standard of intelligence. He found the Doctor particularly annoying because of his cheeky attitude and his obviously superior intelligence. Luke joined forces with the Sontarans because they seemed to appreciate his genius, leading him to commit global treason. In the end the Sontarans turned against him and Luke redeemed himself by sacrificing his life to destroy them. (DW: The Sontaran Stratagem/The Poison Sky)