I can admit that the concept is what absolutely blows my mind the most with this extremely short drama!
There are no words to describe how much I love it. It is, in fact, so interesting and has so much potential for further development, that I'm willing to overlook a lot of the rather unfortunate issues this drama has.

It is a very short, very small (in more ways than one) drama that completely took me over.

The production was clearly tiny, there are about two sets and some outside locations, the cast is really only centered around two people and each episode if only 15min long.
However, those 15min carry a punch! The concept between Mono's and Probe's is fascinating, especially since Choi Yeon Woo's fear of falling hopelessly obsessed with the one person who can make him see colors in his so-far gray-toned world is juxtaposed with Go Yoo Han's complete willingness and desire to be obsessed over, since he in turn is already infatuated with Choi Yeon Woo - the only person who's face he can recognize.
It's such a great concept and even though Go Yoo Han's actor Hurhyun Jun is still a newbie who was somewhat flat in some scenes, Yoo Jun's acting hit it out of the park in every. single. frame.
He was absolutely fantastic as the extremely conflicted Choi Yeon Woo, who is terrified of becoming obsessed with his Probe and his assumption that such obsession can only lead to tragedy.

There are some issues with the story that can't be overlooked - Choi Yeon Woo's mother has been missing for years and this is a plot that is supposedly the framing device of the lovestory forming. However it gets relegated to the sidelines and later forgotten all together. Choi Yeon Woo's suicide attempt gets exactly ZERO reaction from anyone, not his aunt, not his supposed love interest. I liked the plottwist that instead of hurting Go Yoo Han, Choi Yeon Woo instead decided to hurt himself so he can never hurt his Probe, but that whole aspect of the story got no further development at all and it literally felt like his suicide attempt had never even happened. He is also left completely alone with his fear of obsession and nobody, not even his aunt, tries to explain to him that a tragic outcome for a Mono/Probe pair is not a guaranteed ending.

It felt like many aspects were underdeveloped, unexplored or just thrown in at the very end to fill some holes. All in all the biggest problem this drama had was it's limited timeframe.