The seventh Doctor probably changed more over time than any previous incarnation. At first an endearingly clownish figure, he evolved into a much darker, manipulative person as time went on. For a while after his regeneration, the seventh Doctor was highly prone to sprinkle his conversation with misquotes. "Time and tide melt the snowman" and "Waste net, want net" are only a couple of examples of this trait, which eventually vanished altogether.
He is highly energetic, often dashing about from place to place in order to assemble some plan, working quickly and frenetically, using his umbrella as a tool to snag or prod things. He also seems more likely to work haphazardly, and to trip up, either stumbling over his own feet or dropping things. Both of these traits settle down over time, as well, although he never loses his hurried pace.
Initially, the Doctor remains in almost constant good spirits. His only vitriol comes out when he is confronting a baddie. As time goes on, he becomes more brooding and occasionally sullen, and may snap at his companion or friends if they interrupt him during something important, or if he thinks they are being stupid.
After a time, the mainstay of the seventh Doctor seems to be his fondness for manipulation, of both people and events. He may arrange details of an adventure before he actually has it, thus making the way easier for himself or providing a quick solution to some problem. His companion Ace tends to be on the receiving end of a lot of manipulation - the Doctor once returned her to the site of a childhood fear without her knowledge, and later cruelly shattered her faith in him so that a villain could be defeated. The Doctor sees these events as being beneficial in the long term, but the people who are immediately affected by them do not necessarily understand.
If there is anything the seventh Doctor resembles, it is a magician. He has an amazing talent for practically mesmerizing people with his words, or literally with a stare. He can talk people into doing things for him, or talk them out of doing things which might harm him, and they will readily comply. He acquires the new skill of stunning people with a single finger applied to the forehead. Sleight of hand is at least a parlor trick for him, or at most an extremely useful talent.
This Doctor also has an extremely flexible face, and is fond of contorting it in various ways. When he is desperate to get his point across, he bellows his words. He also plays the spoons quite well, and learns to juggle. He enjoys amusing babies and adults alike with his ability to imitate bird calls.
Physically, this Doctor is somewhat shorter in stature. His eyes are typically blue, sometimes grey, and he has an upsweep of dark hair. This regeneration has also acquired a slight Scottish accent, most noticeable on words containing an "r" sound, which he trills with rrrelish. While initially displaying a fondness for plaid, the seventh Doctor later seems to prefer paisley. He wears a question-mark pullover over his shirt and paisley tie, with checked trousers and spats, a cream jacket (later brown) with paisley scarf, and matching hat. As an accessory, the Doctor carries a black umbrella with red question-mark handle.