Post S6 of BtVs, and AU after HBP. Spoilers included.
Also, neither of these worlds belong to me, I'm merely here to play for a little bit.
His hands reached deftly to stroke the bright red hair of his companion, her head tucked in the nook between the arm and the back of the worn sofa. Her eyes closed, her breathing soft and gentle, he smiled at the sleeping woman before him.
There had been no doubt in his mind the first day he met her that there was something more powerful in her than she could ever realize, and that drew him to her. The magic that flowed through her veins was as strong as anyone he had encountered, maybe even the Dark Lord's, but that didn't mean anything to her. She was too shy, too quiet, too broken to use it to her advantage again. He knew the darkness that lay within her, struggling to break free. He knew that it had, once. That's why she was here, now, with him. She had tried to end the world, and had come very close to succeeding. He knew that she would never forgive herself for it. He had seen her struggle with it every day he had known her.
"A mentor," Rupert Giles had told him.
She needed guidance, and he needed a place to be free. The English countryside had been his choice, and he had been more than loathe to the idea of sharing the solitude with someone else.
But he owed the old friend a favor, and had agreed after much deliberation.
When Giles had brought her to him, her eyes had been empty. There was no life left in the girl, and he knew that every moment was one of self-loathing for her. After two long days with her, he knew why Giles had chosen him over the multitude of covens and mentors that had been closer to her home of Sunnydale.
They were the same. No, he wasn't a young woman who had lost her lover and jumped off the deep end out of grief. And she wasn't a double agent scarred by his actions from a war long over, content to live a life of non-existence over constant recognition and hatred for his deeds. But they had both been to places in their life they could never forget and could never forgive themselves for.
When the war had ended, and his true loyalties had been revealed, he couldn't escape from the constant barrage of attention, much of it claiming that he took the side he knew would win when the end came. There were only a few who defended him, knowing that his actions had been necessary. He still had not found the strength to forgive himself for those lives he had taken, the innocents that had been tortured. So, he disappeared. He was still in contact with a few old colleagues, and some of the members of the Order after all these years, but he preferred the quiet of the villa and absence of companions.
When she had come, she tore his world inside out and flipped it upside down. She was a shell of a human being, and despite everything that told him to let her be, he could not resist the urge to try and make things right for her. He was nothing if not persistent. It wasn't until a week after she arrived that she spoke to him. A quiet 'Thank you' had been murmured to his retreating back after bring a tray of food up to her room, even though the previous meals had been returned untouched. She had finally emerged from the guest bedroom a day later, curling up in a large armchair near the fireplace. He brought her tea, which he noticed she took very weak and with too much sugar. He knew better than to try and force conversation with her. When she was ready, she would be the one to speak. He instead spent the night on the sofa opposite from her, reading a potion journal, glancing up to find her staring into the fire.
Day by day, she seemed to grow more comfortable around him. After a month of the quiet that had constantly pervaded the small house, she finally turned to him one night.
"Why are you so sad?"
The question had taken him aback. If anything, he should be the one asking her that, even though he already knew the answer.
"I'm not a good man, Miss Rosenberg. I've made many mistakes in my life, almost all of which I will never be able to atone for. Many lives have been lost because of me, and there is no way for me to ever forget that." He answered her truthfully. His words had surprised himself when he thought about it later. She had surprised him again, not questioning him any further, but rather just accepting his answer.
After that night, an air of understanding stood between them, and Willow Rosenberg began to come to life. He felt like they were making progress. Soon after, she allowed him to teach her how his society functioned. He taught her the basics of many subjects, never asking her to perform any magic, knowing that it would be too much for her to handle. 'All in due time,' he thought.
Friendship was not something that Severus Snape was used to. But the relationship that had blossomed between them couldn't be described as anything else. He enjoyed her company. On her good days, her expressions and odd vivacity could bring a smile to his face. On her bad days, he knew to just be there for her, to yell at, to curse at and to hold her when she finally broke down.
He knew their time together was limited. Giles had mention so in their regular correspondence, wanting to eventually bring her back to her world, to face her friends. Every time he thought of the watcher returning to take her back, his stomach clenched and his heart felt empty. He knew his feelings for her exceeded that which was appropriate, given the circumstance by which she had come to him. But love was love, and it was not something Severus could control.
Incoherent words passed her lightly parted lips, and her brow furrowed in her sleep. He smiled at her, gently removing the ancient runes book from her hands and summoning a blanket from across the room. As he tucked the warm cloth around her small frame, he leaned forward to place a kiss on her forehead.