Well, I really enjoyed the manga so far, so much so that I went looking for the novel and read it all in one day and created this account to leave my general comment. Others have already talked a lot about the story, so I'll talk about Mimi and I'll give a big spoiler, so pay attention!
Spoiler:
In fact, Mimi is the protagonist's former friend, reincarnated as a monster to protect him and also the cause of his suicide.
What happened was that despite being childhood friends, they followed different paths in adolescence and went to different schools, where she got a boyfriend and he, for some reason, didn't know about it (it could be for various reasons), and he ended up catching them in the act, and because of that, he killed himself.
But even after her death, she continued her life and had children and died of old age. However, in death, she regretted not being able to help her good friend whom she loved (like a brother or with slight romantic feelings), so she was given the offer to be reborn and protect him.
Even though she gave indications that she was different from the others and that she still loved him (I don't know how much), she didn't get sexually involved with him but continued to protect him, as he had done for her in the past, even hearing him with other women every night.
Well, she's simply an example of how life can be chaotic even when you do everything right.