Before SHIELD, Natasha was alone. But alone was the only thing she knew, and she was happy with that. She didn't realise what loneliness was when she went on missions, all she knew was how to kill. She was fast, efficient, cruel, and ruthless. Her targets were often cruel and ruthless, but hired henchmen for their dirty work, making her job even faster. Sometimes, her targets tried to match her in combat. But very rarely, someone managed to match her in hand-to-hand combat, managed to land a few blows, managed to do some damage.

And she met two of those people on one day.

The first she encountered also brought along his friend, the crowbar. He landed a few heavy blows, bringing Natasha to her knees numerous times before she managed to break his neck, only realising how much everything hurt when she felt herself collapsing to the floor shortly afterwards. The second man had stood above her, bow and arrow drawn as she lay on the floor, injured, with blood pooling around her.

One thing that surprised her about this man, however, was that instead of releasing the arrow he had pointed at her chest, or settling down to watch her bleed out, he loosed his arrow, returned it to his quiver and slung his bow across his back, instead pulling a medical kit from a pocket on his vest.
"Trust me," was all he said, and in that moment, Natasha felt a flood of emotion crash through her body – never before had she needed to trust someone, and never before did she feel as though someone was looking out for her. She was no longer alone.

As Natasha lay on the ground, watching as the kind stranger with the bow and arrows work over her, she began to wonder who the man was to be kind enough to save her from dying a slow and painful death. Had she been able to maintain a higher level of consciousness, she may have noticed the SHIELD logo emblazoned on the right side of his vest, but as it was, it was all she could do to lift one arm and let her fingers brush the strangers face.
"Who are you?" She managed to croak out, fighting the black spots swarming across her eyes.
"I'm Clint," the stranger replied, pausing from his work at stemming the blood flow for a moment, watching her face, looking into her eyes. Just as she began to fall into unconsciousness, she heard the man speak again. "Trust me."

…..

The next thing Natasha remembered was waking up in a dark room, the smell of disinfectant reaching her nose, the dry feeling of being asleep for a long time in her mouth. Stretching slightly, aware she was sore, Natasha waited for her eyes to adjust to the darkness.

As she looked around, she noticed that she was alone, so she took the opportunity to inspect her injuries, which was, in hindsight, easier said than done, considering she had been in a fight with a crowbar.

When the initial wave of pain had subsided slightly, Natasha realised that her left arm was in a sling, and she could feel the crepe bandages on her arm, the bandages on her legs and face.
"Right," she said, carefully trying to pull herself to a sitting position, aware of the black spots making a comeback into her vision, "no more fighting with crowbars." She groaned lightly as she pulled herself into a half sit, resigning herself to that position.

The door cracked open, and the stranger, Clint, she remembered, stepped in, quickly closing the door behind him, ridding the room of the light that was bright to her eyes.
"That's probably a good idea," he said, a smile touching his lips as he sat down in a chair near her bed. " How are you feeling?"
"Like I got hit by a freight train," she replied in all honesty, wondering somewhere in the back of her mind why she was showing weakness to a man who was, for all extensive purposes, a complete stranger. "Where am I?" She asked, hoping to regain some of her usual, emotionless self, burying her pain and confusion and whatever that feeling of not being alone was somewhere in the back of her mind.
"A secure SHIELD facility," Clint said, noting that she already seemed different from the few seconds between her first and second sentences.
"And I take it you work for SHIELD, then?" Clint nodded, and the room slipped into silence as Natasha took note of the weapons he had on him, and the closest weapon she could reach if need be. Somewhere when she was making these decisions, she slipped back into a sleep, her breathing evening out, her face relaxing from the hard line it had taken when she had buried everything.

From that point onwards, Natasha rarely felt alone again. Even when she and Clint were sent on different missions, or sent on solo missions, Natasha refused to let herself feel alone. When Loki took over Clint's mind, when Coulson told her that he Clint been compromised and a wave of panic engulfed her, she buried it, knowing that everything would be alright, and that Clint wouldn't leave her alone, because he had promised her.

And because she trusted him.