The Ninth Doctor seems to be a rather dark character with a reckless attitude towards danger and life in general (most likely the effect of the outcome of the Last Great Time War).  He could be classified as a 'loner', preferring not to have company and sounding awkward in his invitation to Rose to join him on his travels.  He pretends to be offhand about her travelling with him but ends up developing feelings for Rose, putting himself in danger just to save her.

This particular incarnation was far more informal than his predecessors.  Like the fourth Doctor, the ninth Doctor would make jokes in the face of danger but become grim and serious when left alone.  Like the sixth Doctor, he tends to be fatalistic about near-death experiences.  The ninth Doctor is often impatient with humans, referring to them as 'stupid apes' and dubbing Mickey 'Mickey the Idiot'.

I have noted that the ninth Doctor's emotions seem to be almost all over the place which I attributed to after-effects of the Time War.  He can be deliriously happy, extremely fatalistic, moody and morose, vengeful and angry in the space of a heartbeat.  However, his emotions are more outwardly displayed and almost more violent than the tenth Doctor.  The tenth Doctor is almost calm compared to the ninth Doctor.

As well as this, the ninth Doctor does not shy away from using brute force to achieve his aims.  However, when it comes to destroying himself in order to destroy his most hated enemies, the Daleks, in 'The Parting of the Ways', he finds that he is too cowardly to destroy himself to save the universe.  He does not seem to be able to kill himself for the sake of other people.  He probably fears a similar result to that of the Last Great Time War.  As such, he does not want to see any more massacres, no matter whether it is friend or foe.

Since the Doctor has just become the sole survivor of the Time War, he has demonstrated an extrememly vicious, merciless and vengeful streak against Daleks, shown in the episode 'Dalek'.  So much so that even the Dalek thought that the Doctor would make a good Dalek with all his hate and anger.

The ninth Doctor has a habit of exclaiming 'Fantastic!' when events are going to plan.  He is best known for his leather jacket, shaved head, monkey-like ears (which he himself comments on in 'Rose'), dark-coloured shirt/vest and black pants.  Altogether, a rather darker personality than previously seen.

Thanks to http://en.wikipedia.org/wiki/Ninth_Doctor for the basis of the information displayed here.  Click the link for more information.