Friend
Sherlock Holmes didn't have friends. He had never had friends. He didn't understand the concept of friendship or even the need for such. Why would he want the hassle of catering to someone else's whims? Alone was what he had, and it worked very well for him, thank you very much.
What he needed was a flat mate, someone to share the cost of the somewhat pricey Baker Street apartment. All he required of the man was to leave him alone and to come up with his portion of the monthly rent. It should have been simple. It wasn't. The first three applicants took a look at the clutter at Baker Street, listened to Sherlock talk, and had walked out the door never to be seen again.
Then one fateful day he was introduced to John Watson, who needed a place to live. John didn't think Sherlock's observations were annoying. He said they were brilliant. He was willing to accompany him to crime scenes. He tolerated his experiments, mostly.
Their association flourished and developed ways neither man imagined. Sherlock provided danger and action to the stimulant starved ex-soldier. John in turn, became a moral compass and a conductor of light to the genius. Most surprising to both, they became friends. It was a friendship so deep it became an unbreakable bond.