A weak cowardly boy who is constantly being bullied until he encounters his new teacher using martial arts to defend himself. Shi-Woon begs him to teach him. After several instances where the cowardly boy shows he has heart and will fight with all his might, he becomes Chun-Woo's pupil.

He was given the Illwallsidan (One Moon Divine Dan) by Shi-Ho that forcefully increases Ki, and, had it not been for Goo-Moon-Ryong's help, would have died due to large of amount of Ki his body could not contain. Due to ingesting the pill, Shi-Woon now possess quick healing abilities, greater stamina and physical ability. Though the main effect of the pill is the extremely large amount of Ki he now holds in his body, seemingly giving him endless stamina and great strength in martial arts techniques. However initially, Shi-Woon's body is not yet able to take the strain of the great amount of Ki he suddenly possesses and when summoning too much strength would harm him. Over the course of the first series, he shows that he has an affinity for learning martial arts, as he was able to perform a high level technique, Lightning Steps, after being shown only once. He was also able to align his Ki with external movements, allowing him to use Spirit Breaking Strike within a week. These abilities usually takes months, if not years, for high level Murim to learn and perfect, proving that Shi-Woon may have hidden talents yet to be fully realized. Due to his master's intent of removing him from the Murim world and not suffering the same pain he felt on his master's sacrifice, Chun-Woo destroyed his Ki-center, effectively taking away his ability to do martial arts. Shi-Woon later wakes up in a hospital and learns that after he became unconscious, Chun-Woo disappeared with the other Black Forest members and So-Sul, having only left behind a cell phone with video tutorials on his techniques for Shi-Woon. A few weeks later, Shi-Woon is at school and adjusting back to life before he met Chun-Woo and the others. News has spread of his beating of Chang-Ho and he was being challenged by others for the top spot. On his way through the school exit, he is greeted by a large number of men in black suits led by Gyu-Bum who tells him that he has become the new Head of the Sun-Woo Clan, as he is now in possession of the Phoenix Medallion( given to him by So-Sul in chapter 37 of Breaker) and that a new Murim world awaits him.
The Breaker: New Waves

No longer a member of the martial arts world of Murim, Shi-Woon Yi is supposed to return to his normal life. However, he was given a medallion from So-Sul that names him as the new leader of the Sun-Woo clan. The Sun-Woo clan elders send Jin-Ie Kwon as protection for Shi-Woon, and to make him accept the position of head of the clan so they can make him their puppet. Shi-Woon is confronted by several individuals with grudges against his master. Despite having a shattered ki center, Shi-Woon chooses to fight in order to protect the reputation of his master.The Elder of the Sun-Woo Clan Jae-Kyu Kwon , realizing that Shi-Woon will not be a willing puppet for them, orders Jin-Ie to kill him, but she spares his life. Not wanting to cause trouble for Jin-Ie, Shi-Woon goes into hiding, hoping the Sun-Woo clan will believe he is dead. Shi-Woon encounters difficulties, however, when he starts getting attacked by the S.U.C. (Soldiers Under Command), an organization of individuals claiming to be "disciples" of the Nine Arts Dragon. They claim that in order to fullfil Goomoonryong's will, they must eliminate his "first student", Shi-Woon since he is the only "official" disciple to Goomoonryong. Believing that they are going against his master's beliefs and teachings, Shi-Woon tries to fight them but struggles. He encounters a mysterious homeless man named Bae Seung-Jae who gives him a special medicine that, when consumed, allows Shi-Woon's body to produce Ki for a limited period of time despite his shattered Ki center. With it, Shi-Woon is able to utilize his old techniques and defeat several of his attackers from S.U.C. Told repeatedly that there is no way to fix his shattered Ki center, Shi-Woon resolves to find a way to get stronger.It is recently revealed in New Waves that Shi-Woon does have skills for martial arts, the reason being that he is able to possess a great amount of Ki due to Goo-Moon-Ryong expanding his Meridians (channels of Ki that is stored in the Ki Center) to an abnormally large extent, allowing him to possess the amount of Ki equal to Master class martial artists.
(Source: Wikipedia, The Breaker: New Waves chapter 32,35-36, and 43)

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