DEATHMASTER, the Master of Death!
The name sounds cool, but in reality, she is nothing more than an imaginary being born from the mind of a human girl.
She has no feelings and no life. Her only purpose is to fight endlessly against other imaginary beings such as Black Rock Shooter, until she breaks down and is reborn, bearing the mental anguish of her counterpart in the real world.
She has purpose, but no will of her own.
Now, an outside soul has taken over the empty shell that was the Master of Death. She (or he) cannot accept the tragic fact that she is merely an imaginary being.
And so, the escape begins.