Goo is a master at fighting. He is most likely the best fighter within the Lookism-verse. He uses his skills to intimidate collect money multiple groups across school networks in the area. At one point in the past, Jong Gun was sent to test Goo in a duel. While the victor remains undetermined, it is heavily implied that Goo won, as he gave Gun the scar between his eyes (and possibly more on his back) while Gun caused Goo to need steel pins in his body. The two ultimately decided to partner up in the debt collecting business, eventually making a profit of 20 billion won.
Joon Goo is a tall, lean-built man with glasses and blond hair. He is always shown fashionably dressed, in a similar fashion as Jong Gun. Contrary to Gun, Joon Goo dresses in relatively lighter colors, (white and gold) opposed to Gun's (grey and black).
Joon Goo seems to be very unpredictable and uncaring. When fighting, he can be incredibly violent and sadistic. This is shown when he tells Vasco's bullies that he is a bad guy, just like them, before viciously striking them in the face with a chair. He has not shown any interest in women, however, but has shown huge interest in money. Joon Goo was willing to make some extra money, even if it meant that he was going to reform school because of his actions' consequences.
Lee Eun Tae Arc
Joon Goo is first introduced in Chapter 55. Vasco, Jace, and the "Cicada", 3 fellow classmates, are repeatedly bullied by their seniors for their looks and social reputations. The "Cicada" invites Joon Goo to beat their bullies up for a total of 3,000 dollars. However, Goo lowers the price when he sees the horrific tattoos the bullies have inflicted upon Vasco. Joon Goo manages to knock all 3 guys out with ease and later tells Vasco a "training regimen" before leaving. Later, Jong Gun is seen meeting Goo for the first time, with orders of a test from the HNH chairman.
PTJ Entertainment Arc
Under the HNH chairman's instructions, Joon Goo and Jong Gun beat up a large group of men without much effort. However, Joon Goo gets aggravated when Gun brings up Goo's hard debt collecting job. The two engage in a chaotic fight. Later, the HNH chairman arrives to speak with the PTJ company CEO, who was involved in the Aru scandal.
Joon Goo is seen harassing Logan Lee after his refusal to offer money to Gun.
God Dog Arc
Joon Goo is seen relaxing on a pool chair while Gun stretches on a diving block. Multiple scars can be seen stretching across Gun's back, and it is very likely that Goo was the one who inflicted them. The two partners are talking to Johan Seong, who begs Gun to tell the HNH chairman to move his mother up the waiting list for her cornea surgery. However, Gun refuses, repeatedly reminding Johan to complete his task and claim Gun's position as successor. Goo offers an alternative, which involved an immediate payment of a ridiculous amount of $10 million won.
Jong Gun is Joon Goo's partner and rival. They are seen fighting on multiple occasions. During the Illegal Gambling Arc, Kim Gi Myung (Manny Kim) states that they (Goo and Gun) are a team that makes around 50 billion a year. However, since Jong Gun was hired by the HNH chairman to protect Choi Soo Jung, Gun stopped collecting debts, leaving Joon Goo to do the job all by himself. This infuriates Joon Goo frequently. It is also confirmed during the God Dog Arc that Goo was the one who gave Gun the scar stretching from his right eye to his left.
Joon Goo is one of the ten "geniuses" raised by the HNH chairman. Joon Goo works for the chairman and follows his orders.
When Vasco asked what he needed to do to get strong, he told him the One Punch Man method (100 pushups, 100 situps, and a 10km run every day). Later during the Aru scandal, he was willing to train Vasco as his successor.