I would give 10/10
To avoid the spoiler I will not mention the ending, for me it was very satisfying and I liked it very much. I felt accomplished.

As for the drama, it was kind of plain and nothing much of an original at first look, but everything was so smooth and all the details were out there like a jigsaw pazzle. It goes on and on and you get caught up with the story development and you also start thinking what could it be, questioning the reality and human mind. It was indeed psychological war with psycopath. At first you would think it is not much thriller or something but at some point you might find yourself our of breath.
As story goes on it is getting better and better, the lack of romance - it was not much of a romance but it was essential. For me, everything was its place.

The monotonous tone of the series was ok, it was indeed not your typical Asian drama.
The character development was really good, I think it has well portated how scary it could be if you are targeted by the intelligent and meticulous psychopath. And just having a strong will will not save you. I really did like the story and I wish everybody will watch it- as It seems somewhat underrated.

As for the characters, they were 100% for the role. I fell in love with main leads- not of a fan of girls, but I really loved her. as for main lead ofc he is marvelous. The supporting second leads were soo good.
to sum it up, it was too good to skip this.