There is so much about this show I need to talk about, let me air my feelings out in this not strictly review but review-ish rant about this show.

I have a fair bit to say about the main characters so bare with me.

Yoo Jung
Yoo Jung is the 1st male lead, he is manipulative, cold and distant at best of times, a clear psycopath at worst. He is selfish and petty to the point of cruelty and I don't know why anyone would ever fall for him or why the first female lead would find him interesting at all (unless she was studying his case for criminal profiling). He comes off as the kind of guy who would kill someone because they inconvenienced him.
I mean he actually had some guys beat his best friend up (Baek In Ho) because he told some half-hearted lie about him when he was momentarily jealous of his relationship with another friend. Those guys broke Baek In Ho's hand - thereby ruining his future as a pianist, and Yoo Jung just stood by and did nothing.
I bothers me so much that he not only ruined his relationship with his best friend forever, but he also ruined his best friend's life for a very long time.
He's continually set people up for failure and manipulated his surroundings. One of the most disturbing flashbacks from when he was a kid was when he let a little girl drink alcohol to pacify her and manipulate her into being docile and then he maliciously broke her toy - look I'm no psycologist but that screams psycopath to me.
He constantly hurts people around him and he should NOT be the romantic lead. Or at least, he's not a good rolemodel for what a romantic lead should be. The kindest thing he ever did in the entire show was to break up with Seol.

Baek In Ho
He is the second male lead and the most wonderful guy to ever exist. He's warm, kind, genuinley a good person, he's super handsome and honest. It eludes me why he isn't the 1st male lead. It genuinely astonishes me that he isn't treated better. Now, I like a punky lookin' dude with angsty problems, but it's kind of ONLY suffering for him. I feel so sorry for him that I cry almost every episode. And the real question is; why isn't this story about him?? He clearly has the most interesting backstory, certainly main protagonist-worthy.
Fuck Seol and her school problems and crush on a psycopath. I want a good happy-ever after for my boy Baek In Ho. He's so perfect, bless actor Seo Kang Joon who played him with such sensibility! My heart was stolen by him and I am just 100% watching this JUST to see if Baek In Ho will ever have a happily ever after.

Yoo Jung's dad
I will refer to him as ajusshi from now on. Yoo Jung's dad is as much of a psycopath as Yoo Jung. He's just better at hiding it. He pretty much raised the Baek siblings since their teens, them being orphans were thrilled their best friend's dad wanted them to be his kids. Turns out he never loved them at all and just used them as tools to "normalize" his son's behavior. As opposed to say, actually TALKING to his son about good moral behavior. No, he's leaving it up to a pair of grief-stricken orphans to raise his kid for him!
Again people: HE USED ORPHANS FOR HIS OWN SELFISH MEANS!!
Meanwhile he's pretending he wanted to take them in because he cared about their father as a friend. The notion that he ever cared is superficial at best. When In Ha (Baek In Ho's sister) is publicly humiliated, courtesy of Yoo Jung) he treats her less like a daughter and more like some trash in the streets.
Yes she is loud and obnoxious, but to humiliate someone like that is beyond me, and allow it to happen in front of your eyes with less consideration than a stranger when you were supposed to adopt them is unspeakably cruel.
Him telling both In Ha and In Ho at separate occations that their usefulness has reached an end and that he wants nothing more to do with them is heartbreaking.

Seol
This is supposed to be the main character, but honesly her story isn't that engaging as the people around her. She's not nearly as good a character as I thought she'd be. Yes she tells Yoo Jung off for his psycopathic behavior at first, but as soon as she turns her back he goes and does despicable things anyway - but then half way through the drama she kind of just gives up??
When Yoo Jung sets his overbearing clasmate up for failure at an interview for a job (as retaliation for pestering Seol) she hugs him and says it's ok. Like, what?? It was this kind of behavior that made her break up with him in the first place. But because it happened to someone else it's "ok"??
No to mention that his manipulations in the past led to her being stalked by a guy. And she is mad at him to the point that she puts their relationship on a break. But then eventually forgives him. I'm sorry but being stalked by a guy isn't something a woman should forgive, actually if you meet a guy who had another guy stalk you for ANY REASON, run far, far away! It's not up to him who approaches you, that's a matter that is only up to you. So, for Seol to take him back like that was really disappointing, especially since she seemed to have more moxy in the first episodes.
What really gets me though is as soon as she's back with Yoo Jung she avoids Baek In Ho (who is by now her loyal friend) - like they can't even be friends just because her boyfriend (who should have no say in the matter after the things he's done) disapproves. The look on In Ho's face when he overhears Seol say "I won't spend ANY time with him outside of studying" is frankly soul-shattering, and at that moment I was so angry and disappointed. Why can't she hang out with Baek In Ho as friends just because her boyfriend is a jealous asshole? So yeah, Seol -our supposed heroine has some problems of her own.

Baek In Ha
In Ho's sister may be vapid, inconsiderate and a drama-queen, but she's not evil. She's been treated with as much disrespect as her brother by the foul Yoo Jung and his asshole father.
All she wanted was to be ajusshi's daughter, to have a family (and yeah brag about being rich and spending money) but who doesn't, lol. She's a bit rash and her emotions tend to run hot like her brother's, but she could probably be easily directed in the right path if she had someone with a firm hand to show her he really cares, like say ajusshi. So it breaks my heart that ajusshi just used her like that and he was the one who treated her the worst. He even had her committed into an insane asylum for crying out loud!! My heart breaks for the Baek siblings. They continually face injustice and I just can't..

Summary:
My final thoughts on Cheese In The Trap are that Yoo Jung the first male lead treats Seol the first female lead like shit and she's in love with him for it??
This one is a hard abusive relationship warning!
Main romantic lead is a stalker and a psycopath and possibly a sadist and I just don't get why someone would choose him over a passionate friendly guy (like Baek In Ho) who would treat you well and respect you and love you sincerely. Seriously? What is up with that??
The ONLY reason to watch this drama is because Seo Kang Joon's performance as Baek In Ho is F A N T A S T I C <3 - honestly he's the only reason to watch this show! His character is amazing! In Ho is unfailingly kind, so freaking gorgeous, has this deeply intriguing backstory and I would have been so much happier if this entire story was just a Baek In Ho revenge story of how he got his hand back to playing piano and his road to success.
I mean, I'll give Cheese In The Trap that it at least wasn't boring. But to put a psycopath as a romantic lead is so wrong on so many levels, and then have the girl he's pursuing constantly be hurt physically or emotionally over him is a very damaging depiction of a romantic relationship.