Well, I have many interesting ideas surrounding those two, but I will refrain from sharing them for now.
Oh, and please make a spoiler warning before mentioning spoilers.
My fault,I will make a spoiler warning next time...
And then I noticed that,when Sayuri get near Inei ,he dodged reflexively.So I think that the serum is not simply to create a new personality for the body after Shin's rebirth, but rather to choose a part of the original personality to deal with danger.