Seon Yu is not the strongest of characters, he doubts himself regularly which causes him to be picked on by Owen. He doesn't stand up for himself and tricks Jae-Hyuk into believing he will help him find a teacher; in reality, he was trying to save his own hide from getting attacked by Owen. Seon Yu has a hatred of humans after seeing his father's corpse and learning a human shot him; his father had a love of humans which made the betrayal worse. Overtime this prejudice changes after seeing Jae-Hyuk's kindness towards him.
In Cavalier of the Abyss, his personality undergoes a major change. He is much more ruthless and shows little remorse in harming demons weaker than himself. His emotions toward Regis Nex (Jae-Hyuk) have changed to loathing since he blames Nex for the death of his tribe. In reality, he loses his sanity and goes on a killing spree, slaughtering the villagers, but refuses to admit his actions.
The demon is first shown needing rescue by Jae-Hyuk after tangling himself. Seon Yu was the first demon to show kindness towards Jae-Hyuk; this was a farce so he could lead the unknowing undead to Owen to avoid a beating. Seon Yu does not try to fight back when Owen is outraged with how weak Jae-Hyuk's zenion (magic capacity) is, and runs off when Owen's attention turns to Jae-Hyuk. He hides until he is called to heal Jae-Hyuk; but he proves too weak that Infinty has to step in. He is awed by her powers and pleas for her acceptance to become a disciple.
During Infinity's trial, Seon Yu chooses to find the book himself and take the glory. Once again he proves too weak to handle the demon books on his own and is forced to team up with Owen and Jae-Hyuk. After showing off his magical capabilities he helps give Jae-Hyuk an idea to defeat the book. Seon Yu is resistant to accepting Jae-Hyuk as an ally because of his hatred towards humans; but after the undead shields Seon Yu from an electric attack his hatred begins to waver. They all prove victorious and are accepted as Infinity's students.
Jae-Hyuk and him become roommates, Jae-Hyuk is discouraged by their prisoned- styled room chosen based on their weak power levels. Seon Yu is still hestitant in calling Jae-Hyuk and Owen as his friends. While Jae-Hyuk is sent on outside missions, Infinity has Seon Yu doing physical exercises to build his physical strength since he was already smart to begin with.
When Caladborg is awakened, one of the first places shown that the stone attacks is Urvis, where Seon Yu stayed behind while the rest of the group went off to battle Ren. He questions what is happening to Chaos, when a group of Mahou soldiers approach him and ask for his help.
Seon Yu is taken to a sick Jae-Hoon. The Mahou members force him into healing the boy, who he is unaware was Jae-Hyuk's younger sibling. With the child's body healing, Caladborg easily takes control of the child.
Unintentionally, Seon Yu gave a helping hand in transforming Jae-Hoon into Caladborg.
Cavalier of the Abyss
Seon Yu has a falling out in friendship with Owen and Jae-Hyuk years later. After going nuts and murdering his clan, he becomes a member of Clan of the Night. He believed Nex was the one who killed his race, since he refuses to believe he lost his mind and did such atrocities.
When Xix escapes from the castle and is taken care of by a young girl named Miya, Seon Yu and his partner, One, come to kidnap the child. Xix tries to save her, but his injuries render him useless and the two are captured. The Night members take their hostages to a prison sealed by the Regis. Seon Yu reveals that Miya is the daughter of Nex, Xix's enemy, and forces her to open the seal by threatening to harm Xix.
After the seal's opening, the captors and hostages begin descending further down into the dungeon. Xix reattempts fighting them off, but is once again over-powered. While they are distracted, Yuan and company arrive to save the princess and escapee. Here is where Owen and Seon Yu are reunited.
Owen demands to know what happened to cause Seon Yu to become a Night Clan member. The man bitterly accounts how he falsely placed his trust in a human who didn't really value their friendship. Owen tries to defend Nex and say that he was hurt by Seon Yu's betrayal, but Seon Yu says he is a liar and begins attacking.
Xix and Miya manage to find a hiding place through a portal located nearby. While they are in hiding, Seon Yu manages to stall Owen long enough to chase after the other two. When he finally finds them, he becomes ovejoyed to see that the hostages had found his fiance, Rin, located in the deepest part of the dungeon. His whole goal was to rescue his fiance he reveals and to unlock the Seal. Xix demands to know what other Seal he is talking about, and attempts rushing the man. Having more seasoned fighting skills, Xix is easily tossed aside when Nex makes his appearance.
The old comrades begin arguing about why Nex murdered Seon Yu's race and imprisoned Rin. Nex is sarcastic and replies that Seon Yu knew that he himself was the true murderer, but through his cowardence refuses to acknowledge he is the real culprit. They begin fighting and Nex still proves to be the stronger fighter even with Seon Yu's new powers and magic skills. While they fight, Xix is tossed aside by an aftershock from Nex's attack, and his blood begins to flow across the floor until it reaches the very Seal Seon Yu discussed with him earlier. Despire his loss to Nex, Seon Yu claims he was the true victor as a fragment of Caladborg's soul is released.
The next time Seon Yu is shown he has been imprisoned by Nex. Owen and Mala appear when Xix is sent with Just to protect the other Seal holding Caladborg's soul, Owen joking asks Nex why he responds so coldly to an old friend when Nex cryptically replies Seon Yu considered himself a friend to Nex, also. A small image of a shadowed Seon Yu is seen with him being tortured on his knowledge involving Caladborg.