Biography
It is not specifically known how the Ninth Doctor came in to being, although the commonly accepted explanation is that the Eighth Doctor was killed in the Time War.
He has met three different companions during his lifespan - most notably Rose Tyler, a shop assistant from early 21st-century Earth. He also met Adam Mitchell following his experience with the 'last' Dalek ("Dalek"). The Doctor ditched Adam after the events on Satellite Five, during which time Adam almost changed history ("The Long Game"). The Doctor's third assistant was ex-Time Agent and conman Captain Jack Harkness, whom the Doctor and Rose met on a visit to London during the World War II Blitz ("The Empty Child," "The Doctor Dances"). Captain Jack was 'killed' by the Daleks but brought back to life by the Bad Wolf ("The Parting of the Ways").
The Ninth Doctor was killed by cellular degeneration caused by absorbing the energies of the Time Vortex from Rose, who had absorbed them from the TARDIS ("The Parting of the Ways").
Characteristics
This Doctor seemed manic and full of beans, but shows clear signs of severe survivor guilt, lapsing into sober seriousness at any mention of the Time War.
He spoke with a Northern accent, and was critical of his own large ears.
Habits and Quirks
In stark contrast to his predecessors, the Ninth Doctor wore a plain leather jacket, red or black jumper and dark trousers. He addressed Rose's boyfriend Mickey Smith as 'Mickey the idiot' or simply 'Rickey'. He was also fond of describing situations as "Fantastic!".
He called humans 'stupid apes' and seemed very alien. He carried an air of mystery around him, and sometimes rowed with Rose.
No reference is made to his apparently human parentage (revealed in the Doctor Who TV movie).
Mysteries and Discrepancies
Clive possessed images of the Doctor in Dallas at the moment of John F. Kennedy's assassination, in Southampton just before the launch of the Titanic, and in Indonesia on the day of Krakatoa's eruption. If the Ninth Doctor visited these points in history before he met Rose, why does he react to his reflection in a mirror in her flat in a way which indicates he has recently regenerated? If these visits were after Rose joined him, where was she when these images were executed? Did the Doctor perhaps make these trips after initially leaving Mickey and Rose in 2005, then return at a point just seconds later to invite Rose aboard? The relationship between these events and those in "Who Killed Kennedy," "The Left-Handed Hummingbird," and "Inferno" are also worth considering.
Quotes
(when asked by Rose why, if he is alien, he has a Northern accent): "Lots of planets have a North!" (does this indicate he is from Northern Gallifrey?)
Aliens of London "D'you mind not farting while I'm trying to save the world?"