3. Martin
The third was at least someone up Sherlock's alley, a biology professor named Martin Harrows. John didn't like him.
They both met him during the course of an investigation (luckily he was not a suspect or relative of the victim). A university student had been murdered in Manchester, and he let Sherlock work in his lab. He was clever - had to be if he was a professor, of course, but even Sherlock was impressed - devilishly handsome, and didn't find Sherlock's deadpan attitude at all off-putting. In fact, he found her blatant honesty to be charming and rather sweet. She was instantly smitten.
For two weeks after the student's advisor was arrested, Sherlock slumped and sighed around the flat, staring into space and unable to focus on her experiments. Just when she began eyeing John's gun as though the wall had offended her, someone knocked on the door, and there stood Martin Harrows on the other side, holding a crumpled bouquet of daffodils and asking for Sherlock.
There was a barely-muffled shriek and an echo of pounding feet in the flat as the woman evidently sprinted for her room to change out of her PJs.
"Why don't you come in for a cup of tea?" suggested John with an amused grin, standing back to give Harrows room to pass.
He stayed in London for four days before returning to Manchester, and in those four days Sherlock refused all texts, calls, post...any form of communication that didn't come directly from the lips of Martin bloody sainted Harrows. Two weeks and four days without cases, without murder, without the Game, and John was going mad. His left hand was beginning to tremble again, his leg ached, and he was in an all-around bad mood until it was flooded with guilt. It wasn't Sherlock's job to keep him supplied with bloody fights and adrenaline. He was probably just jealous that she was in a relationship, and the last woman he'd seen was Sarah, which ended abruptly after the Pool.
When Martin left, Sherlock moped for a full three hours before dragging John to Scotland Yard for the case Lestrade hadn't stopped texting her about through the past four days.
John waited a week before asking why Harrows hadn't called. When he did, on the pavement outside of their most recent successful arrest, she shrugged a bit limply and hailed a cab.
"We were too similar," she explained over curry in a steam-filled shop, ears and nose still pink with cold from outside. "I enjoyed talking to him, but there was nothing to give, or to be gained. When I'm with someone, I want to be able to educate them." At his raised eyebrow she sighed. "Not like that, John, not like a professor or anything crass. It's like with you, isn't it?"
He wasn't expecting to feel his heart jump. "What?"
She eyeballed him with her signature don't-be-such-an-idiot stare. "When I met you, you had a psychosomatic limp, a tremor, and a therapist. Within three weeks all but occasional comments on your blog from Ella were gone. I helped you. And you..." She gestured vaguely with a forkful of rice, spilling grains across the tabletop. "You ground me. You taught me Good and Not Good, and make me keep my body parts in the mini-fridge, and you even got me to take that case from Mycroft last month, remember? I had to be in a very small office with him for two hours, and I didn't even hit him. I both give and gain in our relationship, and I want that in a romantic relationship as well." She ducked her eyes, still pink around the ears, and took another sip of beer.
"You...Sherlock, you do realize that you've given me a lot more than my leg back, don't you?" asked John, his throat oddly tight. Sherlock's bright eyes snapped up to his, uncertainty dwelling there in only the way that he was able to recognize. "I now know the names of fifty poisonous plants that you also happen to have in the back garden of your family estate."
That got him a roll of eyes and wadded up napkin to the face. He snatched away her hand, grinning, before it could retreat under the table. "I know how long it takes for fingers decompose at 3 degrees Centigrade, and what sorts of acids are capable of burning through the carpet on the stairs, and exactly what it takes to make a self-diagnosed sociopath laugh. I've learned a lot from you, Sherlock."
Staring at the table, she smiled to herself, just a twitch in the corners of her lips, but it was enough to make John feel warm as August on that cold December night.