The first time she meets Uchiha Sasuke is startling and full of unease. She was subduing a captive in the sixth sector when she sensed the rapid approach of an unidentified chakra signature from behind her. Her body reacted on instinct, turning and dodging and readying an offensive attack-
that was easily thwarted before it was even fully initiated. Her neck was tightly secured in the bend of his arm, the shallow pressure at the side of her throat telling of a kunai there, and her back flush against the intruder's chest. She could feel his breath, calm and steady, as he spoke the words that would change her life forever.
I need your help.
The first time she heals Uchiha Sasuke, he is on the brink of death. He's overestimated his limits and grossly underestimated those of a group of rogue nins and she feels fear rise up in her throat when she sees the light start to dim from his eyes. She does not like to display her scars but she swiftly yanks back one of her sleeves, bares her arm to his lips and orders him to bite. He complies, choosing an unmarked area near her wrist and she swoons a little.
Sasuke has marked her. Many people have before him, but none of them are Uchiha Sasuke and none made her feel this happy about having the Heal Bite ability. She has done something nobody else can do for him and, for now at least, her position with him is irreplacable.
The first time she locates Uchiha Itachi, it is in some small one-horse town and his chakra is at the very edge of her senses. Almost out of range. Upping her pace, she wastes no time telling Sasuke they are close and his speed nearly doubles. He's half mad with the thought that maybe, finally, today will be the day he ends his brother. His honor, his clan's vengeance, and validation as a superior shinobi are all within grasp. His face is so full of hope, his chakra so blindingly bright that she falls that much more in love with him.
But then something happens - something shifts, something changes - and her lock on Itachi's chakra disappears without a trace. It's as if he was never there at all, but her jutsu does not lie. He was here. Sasuke is fast, but Itachi's speed exceeds human capabilities.
The look on his face as she tells him she's lost Itachi is blood-chilling.
He's falling deeper and deeper, further away from what he was, who he used to be. The redemption he once chased dissolves into smoke, and hate and wrath take reign of his direction.
But he is Sasuke and he still needs her help, so she heals.
Karin looks at Sasuke, every emotion he never allowed himself to express colliding and exploding under a torrent of tears as he mourns the death of a man he'd hated for so long-
Karin listens to his voice, the smooth, authoritative timbre of it transcended into a regretful murmur as he informs Team Hebi that they are now Team Taka-
Karin feels his chakra, feels how the last of it's warmth and brightness flickers out and frigid blackness takes its place-
and she wonders if she's really helped him at all.