He was a time thief. He'd snuck up to people unnoticed, stealing a few years of their lives that they'd never miss.
He didn't do it for himself because he had no use for people's life years. After all, he was immortal.
The trouble was that he loved a human. Their love was doomed from the start. Humans die.
That's when he started stealing time. He stole it for her.
Now, she was beginning to suspect something. How long a human could live without being suspicious?
Then, she caught him in the act and left him... with time but nothing else.