Choe Gu-sung was Shogo Makishima's friend and right hand man. He was an expert hacker, even able to hack into Sibyl's core and see its true form. He moved from Korea to Japan because he took an interest in the Sibyl System.
Choe Gu-sung was a man with short straight brown hair that hung over the right side of his face. His eyes were strange in that they appeared to be red and yellow in color. However, he usually hid them by keeping his eyes closed most of the time. He wore a blue jacket with a purple shirt underneath and green pants. He had piercings on his right ear.
Choe was a scientific genius and was the one responsible for spreading the rumors around the Internet, which caused Japan to become chaotic for a while. Choe, like Makishima, wanted to see Japan free of the Sibyl System. He cared not for others and had no problem with people dying, as long as it fulfilled his goals. However, sometimes he showed that he could only take things like blood and gore to a certain extent, as seen when he left Rikako Oryou to dissect her victim alone, since he didn't want to witness it.
For the first part of the series, Choe's role was merely being Shogo's trust-worthy assistant. He helped deliver messages and equipment to the various people Shogo had deemed his "toys." In the second season, Choe's role expanded.
When the helmets were first distributed, Shogo had revealed the motive and functions behind the helmet solely to Choe, who even recordered the scene of the victim, Fujii Hiroko, while she was being beaten to death. This video would later be passed onto Shogo, and uploaded onto the Internet, where many civilians began to chat and panic, leading to intense riots, which only grew worse with Choe spreading rumors as well.
As this riots began, Choe made his next appearance serving Shogo some food and a drink, saying he was curious about what was going to happen next, to them and the town. Shogo commented that it was this ordinary part of Choe that he liked. Choe further said that the two of them were like ordinary citizens in a town not so ordinary. Shogo replied saying that the world reminded him of a parody in a book he read. Choe assumed it was from a book written by William Gibson, to which Shogo said it was in fact Philip K Dick; an author whose work Choe had yet to read.
Their conversation carries on with Shogo explaining his personal opinion on books, how he also prefers them compared to e-books. Soon after, the two leave to carry out the next phase of their plan.
They are later seen before a group of men, who Choe says wishes to see the world beyond the chaos Shogo is causing. Shogo replies by saying that should they be such a world, he would accept it, if not, he would accept that too. Afterwards, they all get on a bus and drive to the Nona Tower, while using the riots as a bait to lure the Inspectors and Enforcers away from it. On their way, Choe explains how he was always so suspicious about the nature of the Sibyl System, and began to suspect its power source, thus leading him to the main tower. Shogo comments that he is indeed 'a genius'.
After successfully breaking their way in, Choe hacks into a computer inside and finds two major power sources, and correctly deduces that the bottom floor one is where the core of Sibyl is. Soon, do they realize that Kogami Shinya is coming, so Shogo decides to leave the plan to Choe, while he heads to the top and deals with the Enforcer himself. This is the last time they see each other. Upon splitting into groups, the Choe makes his way down the many secret floors of the Nona Tower, and manages to get the basement. He proceeds to hack into it, while the men accompanying him lay wait for any intruder. However, the one pursuing Choe - who is non-other than Shuusei Kagari - manages to defeat each man he encounters, and the two enemies soon have a brief conversation.
By the time Shuusei makes his way to the basement as well, Choe has successfully broken into Sibyl's core, and begins video taping it. He says that they don't even need to destroy it, since once it gets out to the public, it'll all be over for the town. However, as he states this, the Chief (Joshu Kasei) appears behind him, aiming a Dominator at him. Choe fails to notice in time, and although he manages to fire and hit his target, he himself is killed by the Dominator's Lethal Eliminator mode.
His phone is later taken from what is left of his corpse, and given to Shogo, as a means to show the latter what he had discovered.
- In the anime and novel version, the setting is different for Choe Gu-sung.
- Choe Gu Sung has moved to live in Japan with his family since he was a kid. There was a war in his homeland.
- His eyes were damaged and were replaced with artificial ones just to save his life. There is no special function about those eyes.
- In the anime version, he has only known Makishima for a year and a half. Therefore, he didn’t know about Touma Kouzaburou. While in the novel version, he had known him (Makishima) for about five years.
- Choe Gu Sung didn’t have interest in women. Those women on his lap in Episode 4 were there just because he wanted to kill time.
- Choe's birthday is 29 October 2070. He was 42 years old.
- His blood type is B.