Maybe I'm the only one thinking this, but this s2 is not completely bad but also not completely good.

Story (7.5)
Interesting concept and starting idea. I think this S2 was a little darker than the first, but yeah, it's just right.
As much as I believe I missed many small details, perhaps because the translation wasn't the best, I understood the final goal of this season.
But speaking in detail:
-it leaves me a little disconcerted that, instead of killing (sorry I love him, it's only for plot's sake) or sending Goo Yan abroad, he has been kidnapped and for a whole series nobody spoke about him. And his family? If I don't remember wrong, they were powerful people, right? I don't know, it all feels so strange
-the final it's really good but if I have to be honest, Hyun deserved better. I loved him from the first time he appeared but like, he has gone through so much and all he had at the end was... a meal from Yeon Woo? Are you kidding me?
-the post-credit scene is... a little forced. I predict that in S3 the character of Goo Yan will return, but with a different face (and actor). Using the stratagem of "the face was ruined" or "he has gone through facial plastic" is brilliant for using another actor, but it seems, I repeat, forced. I think this will be explained like (I swear I didn't read the novel, so no spoiler): when the organization find out they kidnapped the wrong person and also find out he was a probe (or maybe they wanted this from the start), the doctor wanted to understand "if we change a probe's face will the mono go thought color rush again?" and then just operated Goo Yan
-I liked the final plot twist where we find out that the aunt always know (or maybe she discovered after the organization fell down?) where Goo Yan was. At least there is one person at his side.

Acting/Cast (8)
I'm not professional when we talk about acting, but I think Yoo Jun and Hyuk did really a good job. Yoo Jun just confirmed what we've seen on s1. He's a good actor and can really express himself (can we talk about the scene where he cries? cheff kiss). Same goes for Hyuk. Maybe he's not as handsome as Hyun Jun, but his bod- I mean his face is so nice. For the others: Lee Min Ji is good like always; Seo Hoo with red hair just hit different, he's such a cutie; I dunno why but Hyo Gi remembers me P'Gun (from 2gether) and I just love this kind of face. Also the way his character just show up and say meaningful things makes me laugh so much :')

Music (9)
Can someone please stop k-bl to use such beautiful songs? If you want to discover new songs and new artists you can just watch a k-bl and that's it. I just want to do a little specification: in the scene where Yeon Woo and Hyun are for the first time into their "room" the song says "Why do you look away whenever our eyes meet?" and then Yeon Woo say "Se Hyun, look at me!" like he already knew how Hyun feels about him. This is just... wonderful.

Rewatch Value (6)
Probably going to rewatch when a decent translation and s3 come out... for other times... no, just rewatch the s1, sorry.

Overall (8)
Maybe I seemed a little critical on this review, but I actually liked this show. If I don't remember wrong there are 3 volumes of the novels, so probably the s3 it's going to be the last. I think it's going to be suuuuuper interesting, so I'm really waiting for it!