Crash Course in Romance sees a major antagonist depart with a shocking conclusion in its penultimate episode, with many emotions and elements of the past, psyche, and trauma leading up to the finality of events.
Ji Dong-Hui, aka Jeong Seong-Hyeon, is celebrity math tutor Choi Chi-Yeol’s manager. Over the course of the story thus far, he’s been one busy and devoted worker, constantly aiding and supporting Chi-Yeol every step of the way.
From keeping track of Chi-Yeol’s absurdly stacked schedule to even keeping his health in check and even attending to his menial and trivial needs — Ji Dong-Hui handles it all.
However, his innocent face entails a secret identity, which is that of a serial killer. Informed by torture and trauma, Dong-Hui’s murderous actions throughout the show finally lead him to his end in episode 15, where he contends with the tragedy of his own life and puts an end to it.
Chi-Yeol learns the truth
After apologizing to Chi-Yeol for his actions the previous day, Dong-Hui is reinstated back into his position as the star teacher’s manager. However, Chi-Yeol only pretends to make amends with Dong-Hui, while he secretly works to confirm his suspicions about him.
Chi-Yeol follows Dong-Hui to his house and the next day, he goes back there while Domg-Hui is supposed to be attending a meeting in his place.
He breaks into his room and sees the maniacal state of it, with all of his photos and newspaper clippings hanging on the walls, as well as the diary that belongs to Dong-Hui, aka Seong-Hyeon’s late sister, Su-Hyeon.
As Chi-Yeol figures out that his manager has been the same chubby brother of Su-Hyeon he’d met all those years ago at her funeral, Dong-Hui prepares to take care of loose ends.
Dong-Hui was preparing to kill Haeng-Seon when Hae-e caught him, following which he kidnapped her and threw her at his house. Hae-e escaped eventually, but only to fall into a coma after she gets hit by a car.
When Dong-Hui learns that she’s finally recovering, he decides to tie up the loose end and finish her for good before she can wake up and help incriminate him. He goes to her room and is about to choke her to death when Haeng-Seon arrives.
She tries to defend herself and Hae-e as Dong-Hui advances, ready to finish her off as well, when Chi-Yeol catches up to him and knocks him over. As Dong-Hui skedaddles, Chi-Yeol runs after him, with the chase concluding at the hospital’s terrace.
Dong-Hui commits suicide
Chi-Yeol confronts him, for the last time, as Dong-Hui reveals to him why he did what he did. He tells Chi-Yeol that he wanted to protect him, since he’s the only adult he and his sister ever trusted, and so he removed everyone he deemed an obstacle in Chi-Yeol’s path.
Chi-Yeol tells him to stop justifying his crime, and turn himself in, at the very least for Su-Hyeon’s sake, who he says wouldn’t want to see her brother like this. However, Dong-Hui hears none of it, saying that Chi-Yeol has forgotten all about his sister, before climbing up the ledge to end it all.
Chi-Yeol begs him to not commit suicide, telling him that he’ll help him get back into the world. However, Dong-Hui, aka Jeong Seong-Hyeon says he has nothing to protect anymore, and that he’s too tired, before falling to his death.
As he falls to his death, a flashback shows him sitting with his sister Su-Hyeon, who talks about wanting to become Chi-Yeol’s assistant when she goes to college. He asks her why, and she says it’s because he’s the only grown-up she can trust, which is why she wants to protect him by his side.