Every time Clint tells Natasha that he loves her, she nicks him. She will glower at him, using the blood dripping down the curve of his elbow as a metaphor for what happens when he speaks those fateful words. "You show that you have a weakness" she illustrates out loud to him.
She is still his teacher and like all the greatest ones, she is relentless in her attempt to carve this lesson into his mind.
When Natasha returns from missions, Coulson notes but does not mention that it is Clint the next morning that is bearing foreign battle scars.
In Budapest, he won't shut up. He sings into her ear throughout the mission and when she has successfully eliminated the target, she punishes him with sharp but short slashes to his back. He makes eye contact with her while she does it.
"Nat" he breathes, as her blade grazes his left shoulder blade. "I love". She hisses and leaves him another scar. He says it again, and she retaliates. His voice grows louder with each one, louder and more confident. Eventually, when she is damn near infuriated, he reaches back and catches the wrist holding the knife. Carefully, Clint leads her pale hand to his chest and places the tip of the knife directly above a certain vital organ.
Her muscles are tense but for some reason she doesn't dare move.
"Nat" he says, letting his fingers slide down her wrist until he has placed them over top her own. "You may as well kill me now, because I will love and I will tell you so until I am more scar than man. What doesn't kill you makes you stronger, right?" She doesn't need to his face to know he's got a half grin etched into his cheeks. "Loving you, Nat" he murmurs, "it makes me stronger".
He has no memory of her pulling back the knife or of himself falling asleep. He just remembers in the morning that he feels so cold inside when he sees that the spot in the bed next to him is empty. Her suitcases are gone, her half of the closet is empty, and the Russian novels she would read in the mornings have vanished. It was like she was never there.
He can feel every new scar she left on him the night before, and uses that as proof that she had been there. He goes to the bathroom, to wash the dried blood away.
On the tiled floor he finds blood red hair, cut away in sharp slashes.
He kneels, and captures a few strands of hair between his calloused fingers. He smirks at his own reflection in the mirror, and he imagines Natasha slicing off her own treasured locks.
"I love you, Natasha" he says to the phantom. She doesn't look at him, but as he grasps those hairs so desperately, and she hacks off another few inches, he swears that he hears "I love you too, Clint".