Sherlock Holmes
"Who would want me as a flatmate?" John returned at Stamford's mention of a flatshare to afford staying in London. Coincidentally, the other man also knew another person in need of flatmate, but who might be a bit difficult to live with.
The name was Sherlock Holmes, and he knew everything about me in a single glance. He knew about the military service, determined my limp to be psychosomatic (and cured it!), and realized I was in need of lodgings without uttering a single word. He deduced my troubled relationship with Harry, that I was seeing a therapist, and where my phone came from as if it were printed across my forehead. He knew I was a doctor, and within a day was dragging me along to crime scenes he worked for the police on. Admittedly, he got the brother/sister thing mixed up, but missing 'Harry' being short for 'Harriet' was understandable. He was amazing.
He was also mad, John thought, certifiably insane. He could go from a foul mood to positively giddy in moments. Likewise, the excitement of a completed case might last him days; other times he couldn't stand the flat by the same afternoon. Time and schedules seemed to be completely meaningless to him. He had no schedule for anything - not even for eating or sleeping. Sherlock would stay up late researching and experimenting, and still be gone chasing some lead before sunrise. Other times he languished on the sofa unmoving for hours, or days.
There was still a lot that he didn't know about the mysterious Holmes. How did he even get into such a line of work? Where did his money come from since he apparently wasn't paid by the police? What was the "childish feud" his older brother referred to?
As time went on, he wondered what the true reasoning behind the flatshare even was. Wherever it came from, Sherlock had money. He even appeared to have been living at 221B Baker Street some time before John moved in. He was capable of taking care of himself, although he rarely seemed inclined to, but Mrs. Hudson did much more in the way of housekeeping than she'd admit to.
He'd think Sherlock was lonely, but he had made it fairly clear he wasn't looking for a relationship. He knew plenty of people, even if he didn't seem particularly close to them. Lestrade admitted to knowing him longer, but not really knowing much about him. Mrs. Hudson seemed the closest, but she was grateful for Sherlock ensuring her husband's execution. Not sure he wanted to look into that one too much.
John wasn't really sure how he, trust issues and all that his therapist pointed out, came to trust Sherlock Holmes. He wasn't even always sure that he should. But Sherlock provided an excitement and mystery that his life had been lacking recently. He was hoping to find an affordable place to live in the city. What he got was a nice flat in the heart of London, a cure to his limp, and an invigorating new view on life. He could hardly complain if it came with an eccentric flatmate.