I've seen worse conclusions to manga even non rushed ones but something about this one really bothers me. After thinking about it I think I"ll call it the "lie twist".
What I mean is when a series tries to have a big twist that amounts to "THe thing you thought was true wasn't actually true because I lied and gave zero indication I was lying". In this case we had no indication at all the heroine was the alien and not herself and that the alien wasn't either unconcious or fully merged with her human host. Even when she was basically alone or there was no need to fully keep up the act she maintained it anyway for no apparent reason.
It's not that it had to be obvious but even her inner monlogues were shown to us as entirely human. This effectively contradicts the idea it was an act at all. But even if you put that aside the fact the series tries to make a twist by turning a well established fact into a lie is the worst kind of twist possible.
A better twist would have been if she ocassionally acted inhuman. Since it was established the heroine and alien MAY have merged then inhuman tendancies leaking out could be seen by the reader as signs of the alien traits manifesting. Then when it's revealed it was all an act the reader would realize that it was actually the alien making mistakes in her acting rather than a merged personality showing alien traits.
It's the worst kind of twist for two main reasons. First it's just plain lazy as anyone can make a twist by just retconning pre-established facts. Surprise the character who has a funeral wasn't dead, surprise the person we've given no indication is evil is actually the real bad guy. The second reason is it's just not satisfying. As a reader I want to feel like what I read meant something at least within the context of the story. But when the writer does stuff like this is makes the story rather meaningless. Nothing matters because nothing is real even within the context of this story. Any fact could be a lie or turned into a twist later and so any investment is wasted. A properly made twist should make the reader look back on the series and realize all the clues that had a hidden meaning to them.
But in this case the twist only made me look back and see all the reasons this twist made no sense.