Parasyte is based on the story of a new life form whose cause of origin is not known. They have been programmed to take over human brain, physically and symbolically.

Story starts with a parasyte taking over SooIn's brain. Because of some exigent circumstances the normal procedure of taking over can't be performed. Hence, the dual personality of parasyte and SooIn have to work together. Jeon So Nee really shines in this role. SooIn's role might have been weakly delivered but she played an excellent parasyte.

ML's lead is kind of supporting role. Nothing to write about.

Any discussion on this would be incomplete without comparing it to the original work. Instead of focusing on journey of protagonist (like it was in the original), this drama focuses on the face-off between human organisation Grey and parasytes.
Since, the original work had a different intended audience, mostly younger audience, they couldn't explore the philosophical aspects of existence. This one does.

Rather than simply existing humans strive to find connections with fellow humans and that is their driving force for existence.

PS: I gave additional +0.5 because of last 10-15 minutes of last episode.