The early and middle part of the manga has a lot of supernatural inkling plastered around, but it's just bait and switch, it is a real psychological problem and a rare one to boot.
It is hard to prove whether or not someone actually contract it as there are plenty of cases where it's all just chu2byou delusion or silly excuse to avoid punishment.
Most fictions about split personality are pretty shounen silly or straight up edgelord masturbation material, but this one felt believable and ended up in realistic and romantic way.
Giving it 8/10, simply because I hate the deep bait the author gave at the early stage of the manga.