This is a pretty good, non-heavy mystery drama. It follows high schooler's perspective, so it's easier to keep up with compared to detective mysteries while still bringing up some heavy topics like bullying, etc. 
 
It's pretty easy to figure out the culprit early on if you watch lots of mystery dramas, but they still leave you guessing about the reason and even cast doubt midway.
 
The occasional romance moments don't disrupt the whole mystery vibe and happen quite naturally, I would say.
 
SPOILER
The only problem is the ending. Ok Chanmi goes from wanting to kill anyone she suspects to forgiving and talking her way with Seok Jaebeom in seconds. Worse, the dialogue makes Jaebeom seem like he is possessed by an evil spirit rather than having a double personality.
 
Police closing Gi Osung's case as a suicide attempt also betrayed the story. It all started with the police prematurely closing Park Woonseok's case as suicide without proper investigation, so it's back to square one.
 
"Gi Osung deserves that" and "Ji Sooheon deserves to live his life" aren't enough justifications. Sooheon could easily escape that one case using self-defense arguments anyway.