The poster caught my attention initially to watch this drama and the plot twist at the ending of first episode got me hooked.
The overall plot is quite straight forward Zero vs XROCKY, which is done through the sub-plots and their connections.
As story goes on we get to know more about each character as well.

Acting is okay, Though the ML's acting needs polishing, sometimes his expressions are awkward, aside from that he is good in the action. the side characters were okay as well, some better than others. (Karen & Kyoka got me more emotionally attached to their characters due to better acting)

Among the 9 episodes, most were good (it's not that it had out of this world twists but it was entertaining and done well), in order to enjoy this drama, you shouldn't take things too seriously, there are things that you might need to let go & ignore in one or two episodes to enjoy the progression of story & its revelations.
Don't take it too lightly either, this drama might make you cry once or twice.

The ending concluded 90% of what it was building to an end and left a 10% hope for a possibility of continuation (I would like to know more about Zero's identity).
If they ever make a 2nd season, I'll watch it.