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They can make themselves look like other people, right? So if we want to prevent them from tricking us... isn't it best to get rid of all the other people they can turn into?
~ Seongbin explaining his plan to fight shape-shifting monsters.
Just what did I do, anyway? You're a loser, so I treated you like one! You got what you deserve. Why is that my responsibility? Listen, Gyuhwan. You think if I didn't pick on you, nobody else would have? Think about it. You're not looking at the underlying problem here. If it weren't for me... someone much worse might've picked on you! So just maybe... you should be thankful that it was me.
~ Seongbin justifying his cruel treatment towards Gyuhwan.

Seongbin Yu is one of the main antagonists in Shotgun Boy. He was a student at Yeongchan High School and the school bully. After a group of shape-shifting monsters attack their retreat, he allies himself with his fellow bullies to kill any human, since they could by a monster as well. Later, he is hit with the monsterization infection by Zero and takes advantage of his new powers to go on a killing spree which includes all of his classmates, One and the MM. He is ultimately killed by One, however, before fully transforming into a monster.


Seongbin is a young man with short, parted brown hair that is shorter on the sides. He has dark brown eyes and some of his classmates find him attractive. He wears a typical white school uniform over his maroon t-shirt. It is unknown what his full monster form would've looked like, but occasionally his skin gained a flesh-like texture when he hardened himself.


Seongbin is an incredibly sadistic and immoral man, who takes pleasure in bullying students, most particularly Gyuhwan. He hates Gyuhwan for no other reason than thinking he's a loser and relentless abuses him "for an experiment" to see how much he can take. He sadistically enjoys the pain he inflicts on others, even engages in to psychologically unnerve whoever he bullies. He's feels no empathy for who he hurts by bullying, even Gyuhwan's sick father. He gets very angry when anybody thinks of standing up to him.

He does, however, carry a level-headed and strict demeanor despite his immoral and violent actions. Most of the students are aware he is a bully, but many of the teachers see Seongbin as an ace student since he is smart and charismatic. He is shown to be very calculative, even in stressful situations like figuring out the bloodlusted Samshik's motives and deduces that it wants him alive. He is also able to remember the layout of a campground after only seeing it once.

As the story progresses, his psychotic personality starts to show, such as when he smiled at the dismembered bodies of his classmates. Seongbin only truly cares about his own survival. He also isn't above sacrificing people who aren't useful and keeping people who may prove useful, even if they are his friends. Not only does he not mind sacrificing them, he smiles at the thought of doing so. He believes that he must murder all of his classmates so the shape-shifters can not steal anybody's identities, although this is probably just an excuse so he can murder anyone he wants. He even resorts to murdering someone who looks like a child.

Look! I did that! Can you do that, big bro?! I don't care how talented you are! Even you can't do that! If dad sees this, he'll love me too! Besides, you're not even my real brother! You're just a figment of my imagination! I don't have to feel anymore guilt.
~ Seongbin after finding out about his monster powers and imagining his dead brother.

Seongbin's monster self

When he starts to undergo monsterization, he is enlightened by his new powers and views it as a gift. He sees it as an oppurtunity to detach himself from his human emotions, including his guilt for killing his brother as a child. He becomes even more violent and sadistic then he already was. He ignores the danger he places himself in or the fact that his friends are horrified by his actions when doing so. He is aware of his cruel actions but doesn't know why he enjoys doing them. When placed in danger, he is still afraid but nevertheless enjoys the excitement. When Samshik offers him protection in exchange that he kill Sanggwon, instead of following through, he attacks the monster simply because it called him a loser. Once his monster powers become active, he is enamoured with his new powers and goes on a killing spree.

Underneath, Seongbin is actually a very insecure coward. This is mostly shown when he was a kid and got insanely jealous that his father paid more attention to his big brother, who loved and protected him. This is also implied since relies on his two goons to do everything for him. During his fight with Gyuhwan, he shows his true self as an irredeemable scum and coward. Once outmatched by him, he refuses to apologize or accept defeat. After careful thinking, he starts crying and realizing he isn't truly happy like he thought he was when getting his new powers, since he has no home to go back too. He is apologetic to Gyuhwan for taking his frustrations on him. It is unknown if he actual felt this deep down or was completely faking, but he did this as an opportunity to lower Gyuhwan's guard and kill him. Seongbin dies with no guilt or reflection on his actions continuing to call Gyuhwan a loser.


As a child, Seongbin desired his fathers love. Unfortunately, he only payed attention to his big brother. Eventually, his brother saved him from a truck, sacrificing himself and ending up in a coma. Seongbin used this opportunity to murder him in his sleep.

In his high school years, he started bullying Gyuhwan Han, going as far as breaking into his house while his sick father was in the hospital. Gyuhwan's friend, Shinyeong, reprimanded him for making him miserable, to which Seongbin said he wouldn't stop unless she went out with him. When she refused him, he decided to make her life hell and turn her into an outcast.

Powers and Abilities

As a human, Seongbin is an athlete and a top student. He is powerful enough to make everyone afraid to stand up to him. He is also very intelligent and able to remain calm in almost any situation. While undergoing monsterization, he develops superhuman abilities such as regeneration.


It wasn't me!!!
~ Seongbin when asked why he killed his brother.
One: What do you feel right now?
Seongbin: I feel like I could slaughter anyone and anything! *laughs*
~ One asking Seongbin about his new monster state.



  • His gang
    • Sanggwon Ahn (formerly)
    • Chanhyeok Park
  • Samshik - Situational Ally (formerly)


  • Gyuhwan's group
    • Gyuhwan Han - bullying and attempting victim
    • Shinyeong Kang - bullying victim
    • Seonyeob - bullying victim †
    • Chayeong Park - attempted victim †
    • Jongpil Shin
    • Jayden
    • Zero - attempted victim
  • One - attempted killer
  • Samshik - former ally turned attempted captor †
  • Sanggwon Ahn - former right-hand man
  • MM Corporation
  • Gyeong-cheol - victim †
  • At least 4 other students - victims †
  • His brother - victim †
  • His father - abusive figure


  • His brother
  • 3 MM agents
  • 5 students (one named Gyeongcheol)


           Logo SH.png  Villains

Sweet Home
Humans: Gangsters (Gapsu Seo | Hoyoen Baek | Hyunshik Lim | Ihyun Jo/Jung Ui-Myeong | Joon Shin) | Choi Yoon-Jae | Jay Ryu | Seok Kim
Monsters: Blind Monster | Eyeball Monster | Protein Monster | Slime Monster | Speed Monster | Starving Monster | Tongue Monster
Others: Evolved Monsters

Shotgun Boy
Humans: Chanhyeok Park | Director Lee | Gyuhwan Han | Mira Han | Sanggwon Ahn | Seongbin Yu
Variants: One | Three | Four | Five | Samshik | Seven
Others: Zero | MM Corporation