"some even called her a demon"

The Black Monster:
The Black Monster is the terror that descends upon the earth every 108 years to kill the Moon Child, who was supposed to fill the world with Holy Bliss.

That's how the legend goes, of course.  In actuality, the Black Monster is Rose. Rose is carrying out this terrible burden because the Moon Child is actually the soul of the God of Destruction. If Rose does not kill the Moon Child and all that come in contact with it, the soul of the God of Destruction will merge with the body in the sky, which is the Moon That Never Sets.



"Rose has been carrying the fate of the world all alone while shedding tears of blood."



Why Rose?:

Many people ask, why is Rose the one who has to carry out such a task?

First of all, Rose is the only surviving dragoon from the Dragon Campaign. This means she's got  a lot of power.  Do you know of anyone else that is capable of whiping out entire villages, single-handedly, just to destroy one person? Rose was the most capable person for the job.

Charle Frahma chose Rose to do this, because not only was she capable, she was willing. Now don't jump the gun and call Rose a terrible person. She fought in the Dragon Campaign for a world free of oppression. And that is what Rose continues to do as the Black Monster. She is protecting the world.

As she had said, she was not going to let the world that she and Zeig saved together perish. Rose's friends had given up their lives in the Dragon Campaign, and Rose did not want it to be in vain.



 "Don't worry, I am carrying on your will. We took this world back together, I won't let them ruin it."





Immortality:

How is Rose alive after all this time? Why is she immortal?

Well, since Rose was the only person that could continually kill the Moon Child,
Charle Frahma gave Rose a special choker that stopped the passage of time for her. It's obviously magical, and the kind of skill required to make such a device is unimaginable.
This allowed Rose to continue her duty as it preserved her body, even as it corroded her heart.  Rose lacked a perception of the passage of time for a very long while after this. She didn't gain it back until she had been traveling with Dart.





"There are two magic elements sealed in Rose's choker. One is to open the Signet of this city, and the other is...immortality."






Guilt:

Yes, guilt. Rose is extremely guilty over the fact that she is the Black Monster. She recognizes it is a neccicary task, and preforms it without hesitation. However, she seems to have convinced herself that she can never be happy because of this. Nay, doesn't deserve to be happy.
As Dart becomes closer to her, Rose becomes more uneasy about it. She is aware that Dart is from Neet, a town she annihilated trying to find the Moon Child.

After Dart discovers that Rose is the Black Monster, the two of them have a sword fight. More to ease tension than anything else, I imagine. At the end of this, Rose asks Dart to kill her. Because of all the hatred built up inside  him, Dart considers it for a moment. But then, he released that killing Rose would solve nothing, and that Rose was not at fault.

Receiving Dart's forgiveness shocks Rose. It was obvious that she was fully prepared to die at his hand.

The forgiveness is a powerful thing, and though Rose is still unused to companionship, it has helped to heal her in a way.






"The Black Monster is dead. "All that's left is a friend that walks the same road as us. "