The main character Riku is a girl from one of the main demon killing families, like a lot of reincarnation stories do is that they place the soul of someone who died in the modern era and place it in an outcast/sickly from another world so this happens but riku decides to sell that soul to a shinigami (think deathnote) to find a place where she ''belongs'', naturally after her father decides to throw her off a cliff because she couldn't awaken the family's demon killing power.
So she washes ashore near a city and tries to find a job lady lady lada she gets chased she runs into a demon who saves her and asks her for her true name (which basically means that she becomes his slave) because he thinks that she could be useful.
then she becomes happy because now she has a place to stay and that's pretty much it for her character progression. Now she's just like one of those side characters in a harem that is willing to do whatever possible to serve the protagonist which is in this case a side character.
Another reason for hating this novel is because the author makes fun of the side characters that blindly and for no good reason fall in love with the MC trope but then the author thinks that it's a great idea to make the MC like that.
I think that this story had huge potential but the author just couldn't grasp it