Jeong Seong-Hyeon, aka Ji Dong-Hui, reveals himself to be the prime antagonist in Crash Course in Romance in a twist that shocks the central characters to their very core. Leading up to his reveal, however, he has raked up a significant body count as a serial killer.
Ji Dong-Hui is the manager of the celebrity maths tutor at the Pride Academy — “Trillion Won Man” — Choi Chi-Yeol. For a tutor as busy as Chi-Yeol, his assistant needs to be the most dedicated and punctual of workers, and Dong-Hui has been that man, and then some.
Seemingly the kindest person with great humility for himself and respect for others, Dong-Hui harbors a secret most bloody of all — he’s a serial killer.
Dong-Hui’s real name is Jeong Seong-Hyeon, and he’s the brother of Jeong Su-Hyeon, the student who committed suicide due to an inner conflict brought about by her mother’s obsession with her academic excellence.
He’s also the same Jeong Seong-Hyeon who killed his mother when he was in middle school, and eventually got acquitted by the court due to unsubstantial evidence. After his sister’s death, Seong-Hyeon turned to the only adult he could trust, the adult he came to know about from his sister when she was alive.
This adult was none other than the star maths tutor, Choi Chi-Yeol. Before her suicide, Su-Hyeon was a cheerful student who really trusted and likes Chi-Yeol, her tutor, and in her diary, she had written all about it. When she was contending with the guilt of the exam leak she was inadvertently made a part of by her mother, she turned to Chi-Yeol, since he was the only adult in her life she could trust.
After Su-Hyeon died, her mother’s treatment of Seong-Hyeon became worse day by day. Reaching his breaking point, Seong-Hyeon finally pushed his mother off the balcony and to her death. Totally devoid of any person, much less an adult he could turn to, he found refuge in the hopes of meeting Choi Chi-Yeol one day and putting all his trust in him.
Mentally disturbed due to his upbringing and all that transpired in his family, Seong-Hyeon had one person to fall back on, the one person that his dear sister deemed the most trustworthy. So three years after he killed his mother, he changed his identity and got himself appointed to Choi Chi-Yeol’s team at the Pride Academy, eventually becoming the tutor’s manager.
Meanwhile, his trust and love for Choi Chi-Yeol would fester into an obsession and possessiveness to the point that he’ll do anything to get rid of those he sees as wishing/inflicting harm upon the celebrity tutor.
Stalker girl
Jeong Seong-Hyeon aka Ji Dong-Hui’s first victim in the chronological order of things is the stalker student who becomes too big a fan of Choi Chi-Yeol. Resembling the makeup of Seong-Hyeon himself, this girl becomes a fanatic about the celebrity tutor.
She desperately wishes to give the tutor the cookies she baked for him, and when she fails to hand them to him at the academy, she forces herself into his house. When Chi-Yeol goes to get her a taxi home, she imposes a hug on him, which a likely friend of hers captures in photos.
She then posts on an online forum that she and Chi-Yeol are dating. Later, she also posts a photo of her seemingly embracing the tutor, fanning the rumors more. She doesn’t just stop there and keeps fabricating stories of their affair. However, one night on a relatively lonely street, she’s killed by a hooded killer who is Seong-Hyeon.
He can’t stand people being mean or harmful to Choi Chi-Yeol and instantly pulls out his metal-ball-shooting slingshot, killing those he deems a danger to the star teacher.
Lee Young-Min
Lee Young-Min is a spoilt brat who hails from a wealthy and powerful person, and even with his poor academic performance and an obnoxious, stuck-up, and arrogant attitude, he gets selected into the All-Care tuition program through his wealth.
After getting kicked out of the All-Care program when he talks down to Chi-Yeol, he plots to have the celebrity tutor reap the consequences of going up against him. However, before he can do any of that, he’s hit by metal balls.
Not long after that, he runs to save his life, climbing the first floor of the academy. The hooded killer, i.e., Seong-Hyeon, kills Young-Min.
Jin Yi-Sang
The person behind ‘Chiyeolsucks.com’ and Chi-Yeol’s ineffective rival, Jin Yi-Sang is also a math tutor at the Pride Academy and his quest for vengeance against the celebrity maths tutor consumes him.
However, Chi-Yeol eventually finds out about his identity and all that he’s been up to thus far. Yi-Sang deletes all his defamatory work against Chi-Yeol from his computer. Before he can even entertain the idea of repentance or plot his next move, Jin Yi-Sang’s killed by Seong-Hyeon.
The murder is witnessed by Sun-Jae’s elder brother Hui-Ja, who reveals all the details in front of the judge at his warrant review hearing.
His mother
Jeong Seong-Hyeon’s mother was like most of the women at the center of Crash Course in Romance, obsessed with her children’s academic success. She tormented Su-Hyeon for her academic performance, and it’s this obsession that leads to her leaking the exam questions beforehand.
Feeling incredibly conflicted by the fact that she was inadvertently a member of this cheating, Su-Hyeon turns to Choi Chi-Yeol, before she ends her life with suicide.
Even her daughter’s suicide isn’t enough for Seong-Hyeon’s mother, who’d now focus all her torturous attention and treatment on him, depriving him of food and denying him humane treatment.
Fed with his mother’s inhuman treatment, an adolescent Jeong Seong-Hyeon ends up pushing his mother from the balcony to her death, and his freedom from the familial bonds.
Also Read: Chi-Yeol’s dark past in Crash Course in Romance explained
