Im gonna be frank with all the experience i got with this type of manga
A - he loved the real girl and the alien had to leave them while wish for a happy end
B - he love the alien parasite, they get new body and happy end
C - some tragic plot twist end that forced both girl persona with alien persona to mix fusion permanently, either share existences or new persona, happy end
Or
D - none of the above
Hope author can get to D ending, or else im just wasting time with different art but same cliche manga