maybe a ghost hunter au?
Cass saw ghosts before she made her first kill, but that was the first time she realized what they were. Before they were just figures that seemed to wander the compound that Cain raised her in. People who constantly wandered the same path and who no one else seemed to notice. After her first kill though, she see's the man's ghost rising up and she understands.
She sees more ghosts when she runs away. Gotham is full of ghosts. No Man's Land is even fuller.
It is as Batgirl that she realizes for the first time that she can help them. She sees the ghost of man she failed to rescue in time. His ghost fades as she delivers his final letter to his wife. She could see that he was happy in that moment. He was content.
After that, she makes helping ghosts one of her missions. He first priority of course, is that no one dies, but she also wants to help those who have died.
She wants to understand that peace they feel when she helps them. There is a blankness in the body after death. It is in an empty thing. A ghost is an empty thing, hollow. In some ways that scares her. In others, she wants to understand it all.
