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I vote for Netorare This is my suggestion With the MC being a relative of the harem like the husband's brother, corrupting not only the women of the harem with the daughters as well (of legal age, I hope). His relationship with each one is different, with no woman being particularly close, but over time he manages to capture each one. There was an accident at MC's house and he went to live with his brother's family, which wouldn't be a problem for his younger and very rich brother. And he would use this opportunity to attack his brother's women. Suggested occupations for MC: Masseuse, Personal trainee, trainer (college or brother's daughters' school) or just a newly unemployed person. And for any story you're going to write, I wish you luck and that you remember to include the female characters' POV.
I think he should learn water, then he would be able to cover it up by saying that he learned it because of his work, I think people would find it strange that a janitor learns fire or lightning magic. And another thing why separate water from ice they are the same thing, I never understood why the authors separate them as if they were different.
Thanks for the chapter! Reincarnates her sister in one of the students, it seems so predictable, not that this is bad, but why not transform her into a magical being or even a magical weapon that will help Rain get stronger, after all she was able to call her own demon, I don't know if in this world it's common for magicians to make contracts with supernatural beings or if they have sentient magic weapons. I don't know if you would also agree to remove her memories, but let her have the impression that she has to stay with Rain at all costs and without understanding anything she decides to stay close to him to find out the cause of this feeling, and she turns into Yandere again.