Joan Watson is Sherlock's "sober companion". Watson is somebody who, like Sherlock, is also quite damaged and has a little bit of a fragile history.

Watson was a successful surgeon, which adds to her complement of skills. She comes to Holmes when she is hired by his father as his sober companion, to assist him remain abstinent after his release from rehabilitation. After her contracted time is finished she remains on after lying to Holmes, telling him that his father had retained her services. He comes to rely on her input and grows to trust her as she assists him come to terms with his life after addiction. After a while, Holmes reveals that he found out that she was no longer being paid to stay as a companion. He offers her a position as an apprentice, telling her how much she means to him and how she assists him to focus. Watson accepts and starts her training as a detective with Holmes.