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-centry Earth. He also met Adam Mitchell following his experience with the 'last' Dalek (Dalek (episode)). After the events on Satellite Five (during which time Adam almost changed history) the Doctor ditched him (The Long Game). The third assistant was conman Captain Jack Harkness whom he met during World War II (The Empty Child). Captain Jack was 'killed' by the Daleks but brought back to life by the Bad Wolf (The Parting of the Ways).
Characteristics
This Doctor seemed manic and full of beans, but on the inside he was suffering from a tragedy. He lapsed into a sober seriousness at any mention of the Time War.
He carried a Northern accent.
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
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?)