Black Swallow is about a girl who was brought up as an assassin, She had no choice in the matter even though she didn't want to be one. She had to keep her hair short, which meant she envied other girls her age. The better she became at being an assassin, the less it felt like she'll ever be able to become a normal person again, but she couldn't help but feel guilt when she had someone else's blood on her hands.
Once, she was sent on a mission to target a bookstore owner. However, on the way, she ran into another assassin from the same organization. The bookstore owner blocked a strike for her, and both of them ended up in prison. She was scared of what he'd do once he found out her true identity, but to her surprise, he recognized her from when she was little. She was overcome with emotion and cried as he gave her words of encouragement, telling her that even if her past was bad, she could control her own future.
She took his advice to heart, and left her assassin past behind. She became a heroine who went by the name Black Swallow and worked to protect the light. Though she didn't know the whereabouts of that bookstore owner, she always thought about him and wondered if they would be able to meet up again.
The truth was that he wasn't just a bookstore owner. He was an ex-assassin too, and had been trained by the exact same organization. That allowed him to see her for what she was: not only an assassin, but one that had a good heart who struggled with killing people. Under the cover of a bookman, he peddled information and skillfully trapped those that tried to target him. Though Black Swallow didn't realize it, he continued to meet up with her occasionally under different identities, helping her leave the organization as well as having drinks with her.