So we figured it out.
Ardere and her sister Blacky are something like shadows/ghosts.
They can go through walls and so. And they're body is able to become one with the shadows.
They eat the pain and the suffer from humans.

The sunlight hurt them and make they're bodys real for a time. So that's the only way to hurt them.
A stupid ghosthunterduo is after the sisters. xD