Leon is a sickly chachaoong affiliated with Bigak, though he claims the only reason he and Ignatio (and possibly others) were associated with Bigak is for their protection, as their lives would be at risk if they were on their own. He did not consider himself an underling of Bigak, but as a devoted friend of Ignatio.
Leon is a slender young man with white hair and pale skin, and is always seen wearing striped pajamas. According to Muyeong, he looks like an escaped mental patient. He is usually seen with multiple IVs in both arms, and an IV stand that looks something like a human skeleton.
Leon has heterochromia; one eye is yellow and the other is blue. Like several other chachaoongs, his pupils are slit instead of round and when he removes his IVs, his skin takes on a subtle snake scale-like appearance. When he fights Muyeong, he also appears to have snake-like fangs.
Leon is very sickly and expects to die at any time, and as such, he has a slightly cynical personality. He laughs at Jagwi's misfortune but, despite this, cares very deeply for his friends—Ignatio in particular. Less easy-going than Ignatio, Leon is still willing to risk his own life for the possibility of helping his friend. He views this as doing Ignatio a favor as Ignatio lacks the drive to kill Muyeong himself.
He is also shown to be fairly sympathetic to others as he is willing to let Garam and Gabi rest before asking them to use their ability to track down Muyeong, since they have already been working for Bigak for some time.
Leon has the ability to create very powerful hallucinations just from meeting someone's eyes. Some people, like Baekjeong, are not very susceptible to the illusions and can easily shatter the effect. But Muyeong's visual ability made him particularly vulnerable. The illusions can be used to distract a victim long enough for Leon to attack.
His most significant ability is the venom that flows through his body. It is not only powerful enough to affect his own health but is strong enough that a person can be affected just by touching his skin. But it is far more lethal if it is directly injected, which is why he bites Muyeong to give him a stronger dose. According to Leon, everyone dies within 2-4 days of being poisoned by him.
Symptoms of the poison seem to be delirium and vomiting dangerous amounts of blood. In addition to this, the venom also seems to slow down the accelerated regeneration chachaoongs possess, making treatment very difficult.
Ignatio - Leon is a close friend of Ignatio. Though it is unclear how they met or how long they have known each other, Leon is determined not to let Ignatio die, even if it means risking his own life. They are frequently seen in each other's company, and play board games with one another (at which Leon ruthlessly defeats Ignatio).
Mano - Leon and Mano do not seem to be on particularly friendly terms but Mano acts as his doctor, caring for him and trying to counter the affects of the venom in his system. Because of this, they spend a lot of time together.
So far, very little has been revealed about Leon's history; Iganatio refers to him as "hyung," indicating the Leon is likely older than he and Mu-Young. He was born with a strong venom flowing through his veins, and this has caused him to have health issues his entire life. How he came to meet Ignatio is unknown but he is currently only associated with Bi Gak for personal safety, rather than because he is loyal to him or agrees with what he is doing. He doesn't actually seem to care much about Yu-Jin or the situation with the second generation at all, except how it relates to Ignatio and their situation with Chau Yoong.
When Ignatio became hard pressed to deal with the situation regarding Mu Young, Leon took it upon himself to find and attack Mu Young, injecting him with his venom in the hopes Mu Young would die. Though his plan appears to have failed, since Mu Young recovered with the aid of his friends, Leon's sacrifce has taken its toll on his health. Because he kept his venom suppressed, releasing so much at once has made him ill and possibly shortened his own lifespan.
Due to this, Bi Gak had his puppeteers change Leon to a living puppet as an experiment. Leon was found wandering the streets by Ignatio, who transported him to a different location and subdued him. Unfortunately, this meant Ignatio had to kill him. Leon's body was last seen being examined by Sae ha so they could determine how Bi Gak turned him into a puppet and, therefore, prevent it from happening again.