Demon Slayer: From the Start, I'll Kill Muzan Several Times Over

Mochizuki Kanon has an unreliable older sister. Her sister was given a mission, only to discover she couldn't complete this kind of salvation mission, so she dumped it on Mochizuki Kanon instead.

Well, fine. She accepts it gladly. She's read the original story, anyway.

Kanon is a demon, but she'd rather people call her a god.

Kanon does whatever she pleases. Her little System isn't very noticeable, anyway.

If she wants to save someone, she will. If she doesn't, she'll just watch.

Author: “Feel free to suggest what you'd like to see. I'll write whatever comes to mind.”