The male protagonist travels through time and the female protagonist is reborn. Each of them has a system that guides them to kill the "villain" the demon lord. They promise that if the demon lord dies, the male protagonist can go home. If the demon lord dies and the male protagonist dies, the female protagonist can return to the world where the male protagonist lives.
As an author, the male protagonist does not know when a villain was added to his novel, or when the original villain became the protagonist.
The heroine was reborn just to avenge her sister, but she didn't want her sister to die at all.
[The identity of the devil will be revealed in the next part or two]
Concept: People are multi-faceted, and we only see one side.