A girl was considered useless in a family of spiritualists because she can't use that power.
Her brother, the one who'll become a hero in the future, is reincarnator (aka the isekai tag source) and this world is like a game he played before.
So he's trying to go for the full harem ending, except he abandoned the girl early because in the game she's forever a weak character.
Unbeknownst to him, the girl's power is DETERMINATION, and after getting saved by a demon general she swore to serve him instead.
Years passed and she's one of the most powerful person on the demon side, and basically spent each arc running into capture targets who she mercilessly kill because they're enemies.
Though I think I dropped it near the end when the story kinda derailed, just remember that the ending got changed after the initial downer ending got a lot of angry responses.