The end

Basically, alter egos were created to protect the host, for example, the clean freak Shiki appeared because of his childhood trauma of his mom's dirtiness or the polite and calm Kazuha appeared in order to cope with his parents that the host Rei can't do.

But after he met Yuuma, his mental complex was extricated little by little that the alter egos are no longer needed and they eventually integrated with the host.

Actually, Rei's spilt personalities are his personalities (they just...split). Now Rei is completely cured and as I said, his alter egos are just actually his so sometimes they show up in Rei's personality instead. They're not gone, just blended with Rei and become one.

I hope this clarify your question.

Thank you so much !! I understand it all now thank you