This story mentally abused me. I was manipulated into shipping the main character, a cute, loving guy, with his cold, evil ex. Because the world is already ending, right? So why spend the last days alone, when you can have a company that makes your heart go faster? When binge-watching this series, I developed exactly this way of thinking. Nope! The main character should have stayed in his library, rather than letting that emotional parasite back into his heart, end of the world or not.
This manipulation was made possible because of how well the actors were cast and how well they were acting: they made the choices of the characters believable and commendable.
Otherwise, the plot has gaping loopholes, such as the whole motive of the pill of death, the appearance of that young boy, the way they met the idol's brother, and, above all, the idea of chaos, as the Japanese (cannot) imagine it.
Then again, I loved the actors and their chemistry was fire. Will probably rewatch the cute scenes...