Joan Watson

"I know my audiences. I told them what they wanted to hear. They're nice people, your family, but they are, at their core, conventional. You make an effort to appear conventional, but I know, Watson, that you share my love for all that is bizarre and outside the humdrum routine of ordinary life. Your family will never understand this, so I gave them something they would understand."
Sherlock Holmes, Ep. 1.10: The Leviathan



[as unspoilery as possible!]
Joan Watson is an accomplished woman who, to the confusion and consternation of her friends and family, apparently threw it all away when she quit her life as a surgeon and found work as a sober companion. In this new line of work, Joan lived with addicts freshly released upon the world from rehab, smoothed their transition to normal life, and helped them in every way in regards to keeping their addictions in check. A far cry from a surgeon, but she was very good at it.

If quitting medicine was one upheaval in her very planned out life, the next came in the form of Sherlock Holmes, the oddest addict she had ever worked with. While his drug of choice had been heroin, his real addiction came in the form of solving crimes, and she tagged along with him on every misadventure and investigation. Despite struggling with him initially, they did find peace with each other, and Joan became so fond of him that when the time came for her to move on she remained with Sherlock, unpaid, out of loyalty. He had been emotionally unstable, liable to backslide, and she refused to leave him when he needed her.

When Sherlock discovered this, he offered Joan a new career - as his protege. This was not entirely selfless, for he had discovered he worked better with her around, but she took the opportunity with only a slight amount of hesitation. Thus began Joan's new life as a consultant detective.

Personal:
Joan comes off as very friendly, able to smile and joke but be serious as the grave when it comes down to it. She can connect with people, and it is difficult to dislike her - she has an earnest desire to help people in need, though she does recognize when ties need to be cut. In regards to her new career she experiences flashes of self doubt, though she finds support in two important places - Sherlock, and her own mother - and can work through it competently.

Despite her pleasantness, Joan is tougher than she looks, having battled her way through medical school with highest honours and, after a successful career in medicine ending in tragedy, found stability through helping recovering addicts. Her steel comes out the most around Sherlock, who she counts as her friend but who is, at times, an unruly brat. After discovering he put himself in a trance at Narcotics Anonymous meetings to ignore the other addicts, she hid a thumb tack in her hand and threatened to stab him with it should he try it again. When Sherlock hired her on as his assistant, her contract stipulated that he still attend these meetings. Joan is a woman of compromise, but she is also not afraid to pursue what she wants.

She is also not afraid of confrontation and will be entirely honest about her feelings and viewpoints; kindly and sensitively at first, and then not so much. She's been referred to as Sherlock's 'better half' by the NYPD, and this is more or less true - while she has his bite when it comes down to it (in fact, she might be even worse), she has sensitivity and empathy in spades.

Comparisons to note, to those interested in the ever evolving Sherlock canon:
Joan is probably one of the sharper incarnations of Dr. Watson to date, or at least, is allowed the room to exercise her intellect. As the series progresses, she is seen solving her own crimes and even making important discoveries for Sherlock (in the first episode she points out something odd that leads to Sherlock solving the crime, which is a taste for what is to come).

However, this version of her is not nor ever has been a soldier, nor does she take the title of 'Doctor'. Despite all of her hard work in that pursuit, it's not something she is comfortable with being identified as, and is usually introduced as 'Ms. Watson' instead, and 'Joan' by most people that know her. Sherlock still insistently calls her Watson, and while this likely began as a way to keep his distance with her, it's simply become her name to him by now.

She's also probably the most stylish Watson ever. JUST SAYIN', GUYS.