It has never been said, in this regard, that you may have money problems and buy top-notch clothing (just as it is important to impress Mi Jin). Jin Sung had known Mi Jin since childhood, and yet he had liked her. He used to accompany her on various extracurricular activities, not out of his interest but rather to spend more time with her. Eventually Jin Sung found a rather interesting box, one of the activities Mi was attending at the time, and as he had talent and liked it, he diligently trained to further enhance his talent for fighting style and may also be able to protect My Jin.
At some point, he made a friend of Johan's while attending the same church he did when they were young. He had already talked about this with Yohan until later in the series, Jin Sung was well aware of his fighting potential since he was in the first and was also aware that Yohan also had feelings for Mi Jin as well.
Lee Jin Sung (Zack) is a tall teenager with a lean, muscular physique similar to Hyung Suk's (Daniel). As a result of past boxing training, his body is still quite intimidating despite neglecting it for some time.
Jin Sung is seen mostly with a serious and severe face when sober and rarely smiles (unless he is with Mi Jin). He has very sharp facial features, which when combined with its high height, easily intimidate most people. Jin Sung has black hair that he can change to best suit his style of dress. His hair is usually combed to the sides with gel, leaving a line on the right side of the head. He also has narrow, almond-shaped eyes which gives a fierce stare to those who look when he is serious, easily shaking his/her confidence. Jin Sung has also worn earrings on several occasions, possibly implying that he has pierced earlobes.
Jin Sung has a very good fashion sense. Even when he wears casual clothes, Jin Sung maintains a very slick and teenage style about him, especially with his regular use of a cap worn backwards and very good looking sneakers when he's outside of school. Even when in school, Jin Sung maintains a standard being a student in the Fashion Department, but he's not to be compared to Hong Jae Yeol in terms of these standards. The only time Jin Sung ever pays serious attention to his dressing is when it's to outshine his peers, mainly Hyung Suk, or if it's to impress Mi Jin.
Even prior to meeting Hyung Suk, Jin Sung was kind at heart despite his poor manners and extremely high imprudence. He had helped Mi Jin on many occasions - one of those occasions being life threatening as her appendix had burst and Jin Sung rushed her as fast as he could to the hospital, saving her. He also would save Yohan from bullies in middle school but get mad at him for running away shortly after, even though Yohan actually ran away in embarrassment because of Mi Jin seeing him in a pitiful state. Jin Sung was referred to as the strongest in middle school. This greatly boosted his ego which, when in conjunction with his very short and bad temper, was the reason for most, if not all, his fights and was also the reason for his violence towards the weak.
After meeting Hyung Suk, Jin Sung slowly changed in ways that he has probably not even noticed, or is even aware of, these changes. Luckily for him, his love interest, Mi Jin, has. He has matured greatly in quite a short span of time as he is polite to even those who are rude to him. He is seen often telling them off on their unjust actions without resorting to violence. Jin Sung has become somewhat diplomatic, as he will try to reason with people preferring to avoid conflict. Since he no longer wants to fight in front of Mi Jin, even when placed in situations where it's understandable to be angry, Jin Sung controls his temper and will even apologize despite not necessarily being at fault. This misleads many to think he's weak, only to receive havoc upon further pushing Jin Sung's patience. Jin Sung's perception of the weak has changed completely and now sees them as people too. He has even shown remorse over his actions in the past towards the weak. He even apologized to Hyung Suk (in his original body) in his own way though. He will scold and reprimand anyone who bullies or makes fun of others simply because they were weak or makes fun of others because of their weakness. Jin Sung's reasons to fight have become far more honorable compared to before, where it was mostly due to his short temper. He currently chooses not to fight for petty reasons, but will fight if someone he cares for has been harmed, mainly Mi Jin. Jin Sung now knows when things have gone too far (in some instances), such as when he explained to Kim Yui's former boyfriend that it isn't okay to hit a girl when her former boyfriend started beating her up because she cheated on him. In the entire series, Jin Sung is one of the most notable characters with strong character development.
Jin Sung is in love with Mi Jin and has been since they were childhood friends. He cares for her greatly and tries to impress her and not to make a fool of himself in front of her. Jin Sung also gets exceedingly jealous when others flirt with her and will teach those who physically or verbally assault her a lesson.
Jin Sung has phasmophobia and was secretly frightened when he and Mi Jin went to visit the 'Child's Mother'. Wanting to impress Mi Jin, Jin Sung was somehow pushed into writing his name on a painting of the 'Child's Mother'. Even before meeting the ghost, Jin Sung was nervous. After realizing that he met the actual ghost, Jin Sung urinated in his pants out of fear and fainted, showing how petrified he truly was.
Lee Jin Sung (Zack) is a very capable fighter who can hold his own against most of his peers in school and was placed in the same caliber as Vasco and Hyung Suk (Daniel). Unlike Vasco or Hyung Suk (Daniel), Jin Sung (Zack) has neglected his training for quite some time as he was confident that his fighting ability was enough to hold his own against any opponent. His pride in his abilities has since been humbled by the staggering amount of formidable fighters around him who possess even more talent than him. Once realising he's no longer able to fully protect Mi Jin, he has decided to resume training.
Jin Sung (Zack) is a proficient out-boxer, having trained since childhood and is considered a formidable opponent by his peers due to his outstanding talent. Jin Sung's (Zack) boxing style as typical of out-boxers revolves heavily on his incredible speed and footwork. He unleashes quick combos, with great focus on jabs and straights, rarely if ever targeting the body. This fighting style reduces the damage his opponents could do to him by keeping the time in his opponent's strike range to a minimum, avoiding retaliation. Speed is Jin Sung's (Zack) specialty and he heavily relies on it with his defence centred around slipping punches. When in a situation in which blocking is absolutely necessary, Jin Sung (Zack) uses the Philly Shell method, one notably used by Floyd Mayweather, to deflect punches with the shoulder of the non-dominant arm. This method creates an angle with one's shoulder where the incoming punch would slide off. Only a handful of fighters can actually keep up with Jin Sung (Zack) and dodge his punches (Park Hyung Suk (Daniel), Yohan Seong). Jin Sung (Zack) has always been noted for his speed by his opponents becoming their reference when judging speed. If the said opponent is on par with, or surpasses Jin Sung's (Zack) speed, they are considered threats not to be taken lightly which is a further testament to the danger Jin Sung's (Zack) speed poses.
As formidable as Jin Sung (Zack) already is, if his opponent proves to be faster than he is, Jin Sung (Zack) would be at a significant disadvantage. Without a speed advantage, Jin Sung (Zack) becomes a significantly less daunting opponent.
Jin Sung (Zack) has seen to be capable of improvement, after taking a bodyblow from Hyung Suk (Daniel), Jin Sung (Zack) has manifested a defence against bodyblows as demonstrated in his fight against Logan Lee. After resuming training, it can be expected that Jin Sung (Zack) would improve at a vastly accelerated rate.
The protagonist, Hyung Suk, first meets Jin Sung on the streets after Hyung Suk, in his original body, had just gotten a haircut. Hyung Suk thought his haircut had made him very handsome, but nobody else seemed to think so. When Hyung Suk finds Mijin looking at him, he thinks that she is interested in him, so he smiles at her. Jin Sung sees Hyung Suk smile at Mijin and thinks he's propositioning her. He confronts Hyung Suk, and may have gotten into a fight if Mijin had not steered him away, telling him not to act aggressively in public. While walking away, Mijin smiles apologetically at Hyung Suk for Jin Sung's behaviour. However, Hyung Suk takes the smile for flirting and asks Mijin and Jin Sung out for coffee. Unable to keep his anger under control, Jin Sung punches Hyung Suk in the face while bystanders took pictures and uploaded them online.
The first day of school, Hyung Suk realizes that his seat is right next to Jin Sung's. When Ha Neul begins flirting with Hyung Suk, Jin Sung tells her to stop flirting because it was annoying.
When Jin Sung sees Mijin staring at Hyung Suk, he begins to hate Hyung Suk for being handsome because he thinks that Mijin likes Hyung Suk.
When Jin Sung goes to the convenience store where Hyung Suk works at night, he is upset because he thinks Mijin is attracted to Hyung Suk in his new body. He tries to extort free alcohol and cigarettes from Hyung Suk despite the fact that he and his friends were all underage.
It was also revealed that Chapter 2 was Jin Sung's date with Mijin. Moreover, Jin Sung had asked Mijin out, but she had refused.
Jin Sung was so upset by all this that he beats Hyung Suk up. In order to ensure that Hyung Suk doesn't say anything, they take pictures of him with his pants down.
The next day, Hyung Suk in his new body walk in on Jin Sung, his friends from last night, and Ha Neul talking about the way Jin Sung had beat him up the night before. Jin Sung's friend was showing off the picture he had taken of Hyung Suk with his pants down. Ha Neul waves at Hyung Suk, telling him that Jin Sung's friend had something fun to show them. Hyung Suk gets mad because they were making fun of him.
When Jin Sung's friends show off the picture of Hyung Suk and compliment Jin Sung's speed, Jin Sung tells them that he isn't proud of hitting Hyung Suk even though he actually wants them to speak up louder so Mijin would hear. He thinks that Mijin would think he is cool for being very strong. Instead, Mijin says that he hates the way Jin Sung fights all the time. She explains that in Chapter 2 Jin Sung had invited her to the movies only to get into a fight (with Hyung Suk). The girl chatting with Mijin tells her that Jin Sung used to be a rising boxer in middle school.
Jin Sung brags about his fighting expertise by explaining that he shouldn't have hit Hyung Suk with a straight punch but instead with a jab.
He and his friends then laugh that Hyung Suk's mother must also be "a pig," and Mijin thinks they are disgusting.
One of Jin Sung's friends then says that he will take both Hyung Suk in his original body and his mother. He goes on to say that Hyung Suk's mother would not have been happy when she gave birth to him and that, if he were her, he would have had an abortion.
Hyung Suk realizes that his mother was being insulted because of him and gets up to tell Jin Sung and his friends that they have gone too far. With this, Jae Yeol, friends of Jin Sung, Mijin and Ha Neul seem surprised, while Jin Sung is outraged because Hyung Suk made a fool in front of Mijin.
Jin Sung then takes a fight with Hyung Suk, raising his own table and telling Hyung Suk to get up.
Outono Boot Camp
Hyung Suk sits next to Jin Sung on the bus to the camp.
When in the camp, after the talent show, Jin Sung and Hyung Suk are invited by Hyun Do to drink with him and several girls from the Department of Beauty. Jin Sung makes an excuse to leave, and Hyung Suk follows. Jin Sung takes Hyung Suk to Mijin's room where they drink with Mijin, Ha Neul, Ji Ho and another girl. Because they all fainted after being drunk, except for Mi Jin, who did not drink a drop, they were not caught by the camp counselors.
Later, when Mijin was nowhere to be found, Jin Sung ran to find her, alerting Hyung Suk that something was wrong. Jin Sung finds Mijin in a shed, crying and bleeding, trying to keep Zeus , the camp instructor who was trying to untie her pants, out of it. Infuriated, Jin Sung attacks him, only for Zeus to seize him. Jin Sung realizes belatedly that Zeus knows Muay Thai, and Zeus is able to break his ribs and both arms. Zeus thinks he has won, but Won Bin suddenly grabs Zeus's leg. Won Bin, unable to stand, crawled to stop Zeus. Jin Sung soca Zeus in the face, despite his broken arms. When Hyung Suk encounters Jin Sung and Mijin, they find Jin Sung out of his mind, repeatedly punching Zeus in the face.
As Zeus is being taken away by the police, viewers comment that he looks too handsome to be a sex offender and speculates on this to be a false accusation. Meanwhile, they laugh at the fact that Won Bin looks more like the perpetrator than the hero. Jin Sung, sitting in the back of an ambulance, confronts the onlookers, telling them to care about their mouths. He bows to Wo Bin, thanking him for protecting him and Mijin. When Mijin asks if Jin Sung is well, Jin Sung energetically replies that he is totally fine and tells her to stay with the police. As soon as Mijin leaves, he collapses. Jin Sung was so injured that it took two months to recover, and Mijin was taken to take apples in the hospital bed and thanked him for saving her.
Eun Tae says he came because he wanted to make people stop saying he's dumb. Jin Sung, trying to act cool despite his obvious fear, laughs at Eun Tae's superstitious ideas, making fun of Eun Tae, saying that ghosts do not really exist. Bum Jae tells him that if he is not afraid, he should sign his name on the glass frame around the picture.
When Jin Sung arrives at the warehouse, he realizes that the lights can not be lit. He then rushes to a glass panel, only to see the portrait of the ghost woman staring maliciously at him, bloodshot eyes wide open. Jin Sung hastily signs his name on glass and skedaddles. As he runs back into the classroom, he cries like a baby, just in case.
In the classroom, Bum Jae is incredulous that Jin Sung signed his name on the portrait. He wants to take an immediate look at Eun Tae, but Eun Tae, scared, tells him to go tomorrow.
The next morning, the school is agitated with the news that Jin Sung has signed his name on the portrait, and they all head to the storage room to see if it's true.
When they get there, they realize that Jin Sung's signature is not in the glass frame outside the picture, but in the windowpane. Jin Sung realizes that the lady's eyes of the picture are closed and that he actually saw the ghost the night before. He faints and wet his pants.
In the end, he explains that if he had hallucinated due to fear or actually seen a ghost, the important thing was that Mijin saw him wet his pants.
Jae Yeol's wallet
When Jae Yeol rushes to the seat of Hyung Suk to teach him a problem, Jin Sung can be seen sleeping next to Hyung Suk in the classroom.
Park Hyung Suk
Despite being in a very strange and brutal situation, Hyung Suk managed to convince Jin Sung, without Hyung Suk to know, to stop bullying others and defend the weak. In subsequent episodes, it is shown that they usually get along well with each other.
When Jin Sung was young, he had made a wish to protect Mijin. As he grew older, his romantic attraction to Mijin never faded, and Jin Sung went so far as to restrain his struggle for her.
- Whenever Jin Sung is drunk, he becomes a weak, weeping face. Once, when he was drunk, he hugged and kissed Hyung Suk.
- Sometimes Jin Sung has Hyung Suk (although Mi Jin's attention goes back to Hyung Suk, although Hyung Suk has no feelings for her.)
- Some have said that an appearance of Jin Sung resembles the famous tattoo maker, Kiji (named after Instagram: Kijikush).
- Zack shares similar English name’s with Zack Lee, an Indonesian/British Boxer
- One of the supporting characters, Ma Cho Il (student of the 2nd Fashion Department), resembles the face of Jin Sung. The difference between the two is his long curly hair, while Jin Sung keeps his fringe of hair brushed with ointment.