A/N: Thank you, I'm really happy you like the Nathan AU. And, damnit, but your prompt distracted me from working on it, so... here's the fill! =)
I hope you like it. (It's pre-slash, although only if you squint).

Their number this time isn't a perpetrator playing victim, like others have been-- he's just a young scared kid with trust issues and a knife that Reese doesn't know is coming until he hears the skid of a footstep behind him.
Ben Daly should have gone for a stab. Maybe a quick dart towards the neck. Instead he throws a wild slash that catches Reese in the abdomen. The air going out of him like a punch without the pain, clatter of the knife and the door clanging as Ben runs out of the room.
"Reese? Mr. Reese, what happened?"
Reese is standing amidst a complex of old warehouses, slatted metal and peeling paint and weeds that don't get enough sun. The kid they've been trying to help had taken a few wrong turns and wound up with a gang that doesn't let members get out; Reese had followed him to the warehouses, planning to deal with the gang before they dealt with Ben.
"Just a scratch," Reese tells Finch-- and it can't be much more, can it? It barely even hurts.
Shouting not far ahead, the clang off metal; Reese rounds the corner fast enough to see Ben being hauled into one of the alleys between the warehouses by six twenty-somethings.
Reese pulls out his gun. Sunlight glinting off walls in disorienting flashes. The structure farther away than he'd first anticipated and his legs weighing heavily, turning liquid.
"How bad is it?" Finch says in his ear. "Really."
The logical part of his brain has started a mental countdown from twenty five. "They've got the kid, have to get to him." Although something in Finch's tone tells Reese that isn't what he asked.
"John, you're hurt, stay where you are." When Reese doesn't answer he hears Finch take that level, steadying breath, and say evenly, "You're in no condition to do take on a dangerous gang."
Reese realizes, feeling nonspecifically pleased, that Finch can tell that he is in "no condition" just from the way Reese is breathing. "Even in this 'condition'," He says implacably, "I'm twice as good as they are." Which isn't really boasting. He pulls out his gun and ignores Finch's "John"-- and that should tell him something, Finch only ever drops his security-blanket formalities when someone is dying-- and slips into the alleyway.
Twenty two.
Ben is sprawled on his side and one of the gang members has a gun leveled at his chest. Reese shoots five of them in the kneecaps before getting tackled by the sixth. He rolls, lashes an elbow to the side and scrabbles to his feet. Reese knocks the man out, quick and efficient with the butt of his gun. Now it hurts. Ben is at the back of the alley and he throws him a wide-eyed look before rabbitting out of it. This time, Reese leaves him be: threat, resolved.
Nineteen.
Reese stumbles into the warehouse. Shelves, equipment-- hammers and nails and screw drivers, tools originally meant for building things until he made a mockery of their original meaning during his work with the CIA-- and a lone tube of superglue. It's crude, of course, but he's done it before and it does its job now.
And he's been here before, too, feeling the shake in his hands and the metallic taste in his mouth and he knows, superglue or no, that yes: he took too much time going after Ben, got his pulse pounding during the fight fast enough to increase the blood loss. "Blood is thicker than water." Reese mumbles, which is amusing at first but then he repeats it again, hard at Finch, for the sheer truth of it. He's dizzy. Dizzy and falling.
Seventeen.
"I'm very close." Finch says softly.
Reese's head jerks up. At what point had he sat down?
He listens hard: on the other side of the connection he hears the low rumble of a car, water slicking against the sides. "You haven't been in the Library for a while now, have you." Reese knows he should be reprimanding Finch for speeding towards him even before the danger had been subdued; he wants to inform Finch of costs and consequences and the fact that he isn't worth it-- but he doesn't. There is something about Finch coming for Reese that is too intuitive to argue.
Fifteen.
The stone is cold beneath him and his coat is crumpled and soaked, pressed against his abdomen. He's always thought he'd go by a gunshot, but bleeding out will do: it's hideously appropriate, and Finch would find it abhorrent but he finds it ironic. Blood. He's spilled so much of it over the years. Swam in it, drowned in it. "Have... have to stop the bleeding." He mutters.
"Yes, John." Finch says, thready and panicked. "You've got to stop the bleeding. I'm close, just..."
He's trailing, fading-- long concentric swirls pulling Reese downwards. He clings to Finch's voice like a lifeline and wonders at what point dying had stopped being the easy part. "Tell me something."
Silence. And that silence spans out long enough for Reese to realize that his version of silence isn't actually a lack of noise so much as the lack of Finch's voice in his ear. Then: "Forgive the-- morbidity of the topic, but you are indeed correct: blood is thicker than water. Roughly six times thicker. The proverb is generally taken to mean that family is more important than any other bond."
Thirteen.
"The second meaning derives from soldiers. Blood pacts that these men forged with their comrades, that the bonds they form with those they fight with are the strongest." His pause is gentle enough that it asks the question without needing an answer. And the grace of that. Reese nods to himself.
Twelve, the doors sway open and it's Finch and the countdown stops. He has a bag slung across one shoulder and he drops to his knees in front of Reese.
"Dear God, John."
Finch stares at him for a moment-- calculating something, Reese is sure, although that means little since the man is always considering something-- and then draws a pair of needles and a thin tube from the bag.
Reese lifts his head. "You have an IV in your field kit?"
"Since meeting you, I've had to rethink my definition of necessary medical supplies." Blood is pooling on the cement and Finch is ruining his perfectly-pressed, bespoke suit. "I think even you'll agree that there isn't time to get you to one of my private clinics." Finch says. He pulls out a bottle of disinfectant and gives it a critical shake. He peeks at Reese and offers him a shaky smile that's supposed to be reassuring. "I am very good at learning by observation."
Easy for a man who built a worldwide surveillance system to say, but Reese thinks Finch has it backwards. He's witnessed it in Finch's eyes after their failure to save Matt Duggan from a bomb, he's heard it in Finch's voice after he'd secured Scott Powell a good job. They learn to live by living, and then they observe; which Finch demonstrates by missing the vein three times before getting it right on the fourth.
They have both begun to live to learn again.
It's only as Reese watches Finch insert a needle into his own vein and connect it to the tube running into Reese's arm that he understands.
"No." He starts to pull away.
"I'm O-positive, you're B-negative" Finch says, latching onto his arm with his polite don't be stupid look.
But that's not it-- Reese can't accept what Finch is offering, can't take a pass-go-free or a get-out-of-jail card. Because he knows all too well what his own blood is made of: it's the dull rush of it through his head as he stands over a killing ground, it's the hot thrum of it under his skin with Kara Stanton.
"It's okay." Finch says firmly, except it isn't.
He thinks of the cooling corpses, three businessmen who had been clinking glasses when he'd shot them, he thinks of the dark rooms he's walked into with wordless things, he thinks of the restaurant server he and Kara had killed after sharing a dinner, and he still doesn't know whether those people had done anything wrong.
Reese bats at Finch weakly.
"Stop it, John, stop--" Finch lets go of his wrist and turns fully towards him, forcing Reese to meet his gaze. Finch is a locked room fortified with steel walls and key codes, but his eyes are opening wide and he's letting Reese see, wholly and completely.
Again: the grace of it. So Reese looks.
When he'd met him by a bench beneath the Brooklyn Bridge Finch had been pale and haunted, still is-- but there are less ghosts in his eyes now, and Reese knows why. He recalls the lives they've saved, Joss Carter's arms opening wide as she runs towards her son, Joey Durban stepping onto a bus with his girlfriend, placing that tiny precious bundle into the Veda Cruzs' arms. He thinks of letting Jessica walk away and he thinks of waiting for Harold in the remnants of an abandoned warehouse.
All those images juxtaposed in joy and grief, clicking and sliding into place like a deadbolt, although he isn't sure whether they're being locked in or being locked out. A hundred broken fragments that don't fit together, except maybe they do.
There's an edge there in Finch's gaze, somewhere past the irises, full of ambiguity and intensity, saying something Reese thinks he already knows.
He stops fighting. The makeshift intravenous line slips under his skin.
Reese feels Finch's blood running into him, warm and clean and strong. The line jostles as Finch eases to the side and awkwardly slides into a sitting position beside Reese, shoulder to shoulder, that tube connecting them in a no-longer-figurative lifeline.
"Hope you're not fond of that suit." Reese murmurs.
"It's replaceable." Finch answers.