The Sixth Doctor's regeneration was initially unstable, and he nearly strangled Peri before he came to his senses. He encountered many old foes including the Master, Daleks, Cybermen and Sontarans, and even shared an adventure with his own second incarnation. Later, the Doctor was put on trial for the second time by his own race, the Time Lords. The prosecutor at that trial, the Valeyard, turned out to be a possible future, and evil, incarnation of the Doctor himself. The events of the trial tangled the Doctor's timeline slightly, as he left in the company of Mel, whom he technically had not yet met.
When the TARDIS was attacked by his old enemy the Rani, the Sixth Doctor was somehow injured and regenerated into the Seventh Doctor; the exact cause of the regeneration, however, has never been revealed on-screen.
The Sixth Doctor is the first incarnation of the Doctor for which an estimate of length of his tenure can be extrapolated from the dialogue of the television series. In Revelation of the Daleks, he states that he is 900 years old; in Time and the Rani, the Seventh Doctor, having regenerated from the Sixth only hours earlier, states that his exact age is 953, indicating (presuming the Sixth Doctor gave his real age earlier) that 53 years had passed (for the Doctor) between Revelation of the Daleks and his regeneration (the exact length of time between The Twin Dilemma and Revelation of the Daleks, however, is not indicated). Given the convoluted circumstances surrounding Trial of a Time Lord, a suitable gap does exist for this. When the series returned in 2005, however, the Ninth Doctor also claimed an age of 900 years, and the Tenth Doctor 903, seemingly contradicting the earlier claim by the Seventh Doctor.
The Sixth Doctor was an unpredictable and somewhat petulant egoist, whose garish, multicoloured attire reflected his volatile personality. He was both portentous and eloquent — even for any Doctor, of whom he saw himself as the finest incarnation yet — and his unpredictability was made even wilder by his mood swings, manic behaviour, bombastic outbursts and glib, unflappable wit. He was also of a fatalistic nature, a trait which reappeared in his ninth incarnation, who had also come into being following dire emotional and physical circumstances.
The Sixth Doctor was almost supremely confident in his abilities and did not suffer fools gladly; he sometimes seemed to endure Peri's presence far more than he actually appreciated it, and his superiority complex applied to almost everyone he encountered. His intellect could support his ego -- for instance, the Sixth Doctor was the only one who was able to repair and operate the chameleon circuit within the TARDIS, allowing it to change shape to suit its surroundings rather than looking constantly like a police box. However, not only did his melodramatic arrogance and caustic wit eventually subside, it actually hid the fact that he had a strong moral sense and a heart of gold (glimpsed in Revelation of the Daleks, in which he showed great compassion to a dying mutant). Underneath his blustering exterior, he was more determined than ever in his universal battles against evil, possessed of a tenacity and a thirst to do what was right that was far more visible than ever before. Despite his often unstable demeanour, he was always ready to act when necessary, and very little — even his companions — could hope to stand in his way.
His condescension towards the universe around him also extended to his companions, especially Peri. While his use of violence and his abrasive relationship with her were both often criticised by fans, the violence was largely in self-defence, and his relationship with Peri had mellowed when the programme returned from hiatus for Season 23's The Trial of a Time Lord.
He was well known for his love of cats, and always wore one of a number of cat-shaped pins or brooches on the lapel of his patchwork coat, itself said to be the height of fashion on a distant planet.
Colin Baker wished to dress his Doctor in black, specifically black velvet, to reflect his character's darker personality. Producer John Nathan-Turner, however, opted for a deliberately tasteless costume with garish clashing colours. He also retained the question marks embroidered onto his collar which Nathan-Turner had added to Tom Baker's costume in 1980 and had retained through Peter Davison's tenure. Baker added a cat badge to the ensemble. Nathan-Turner later conceded the costume was a mistake and detracted from the show.
In recent years, a blue variation of the costume has become a popular alternative. This outfit was initially used in the webcast Real Time as the clashing colours of the original design were tricky to animate.