Are angels and demons destined not to be together?
Not necessarily, maybe, they can break through the boundaries of race and get together for a long time, maybe because the angel likes the devil's beauty and humor, and the devil likes the angel's intelligence, innocence, and kindness.
Are demons destined to be bad guys by nature?
Not necessarily, maybe, the devil is forced by the world, and his nature may be more kind and innocent than the angel.
There are too many uncertainties in this world. Destiny cannot be controlled by others. I wonder how the protagonist in the story develops...