O-Chul is a leader of the paladins of the Sapphire Guard. O-Chul was a fighter before joining the Sapphire Guard and becoming a paladin. He is first seen when he is guarding the throne room just prior the OotS visit Shojo, while the latter was feeding Mr Scruffy. He then was sent to transport the Linear Guild arrestees in prison.
Along with Lien, he is charged with the duty of transporting Miko to her prison cell. During the planning for the defense of Azure City, he agrees to guard Soon's Gate with his life, stating that he and his paladins "will gladly lay down [their] lives in the defense of the tower." On the night before the battle, he reprimands Haley for treating the oncoming assault flippantly, prompting her to spend the time she has left before the battle with Elan.
When Xykon enters the throne room he is immediately met by and set upon by nearly the entire Sapphire Guard, led by O-Chul. However, Xykon had inscribed a symbol of insanity on a bouncy ball and thrown it around the room, which drove nearly the entire Sapphire Guard to set upon each other. O-Chul then realized that to prevent Xykon from taking control of Soon's gate, he would have to destroy it. However, just as he raised his blade to strike, Xykon paralyzed him, just as the last of the Guard regained her sanity and committed suicide (possibly seppuku) out of grief and shame over what she had done while under the influence of the Symbol. Miko commandeered his blade and used it to shatter the sapphire holding the Gate closed. Hinjo believed it was O-Chul who shattered the sapphire and mourned his death, though it was later revealed O-Chul's (still) paralyzed form was thrown clear of the blast and landed in the middle of a tea party held by the Monster In The Darkness, along with the body of Roy.
When Haley and Belkar went to collect dead Roy's body, they found the Monster and decided to rescue O-Chul as well. When he complained about his loneliness, a Demon Roach told him to stomp his feet, that created a series of huge fissures in the ground which Roy falls into. Belkar then threw O-Chul in order to help Haley pull Roy back. O-Chul was then recollected by the Monster, but not before "playing" Monopoly with it as the Banker.
During the three months of Team Evil's occupation of Azure City, O-Chul has been actively tortured by Redcloak for information about Girard's Gate while often being put to life-threatening situations for Xykon's amusement. Redcloak's latest interrogation attempts had him threatening O-Chul to throw groups of prisoners either into the rift or off the tower. However O-Chul has stated that he himself lacks any knowledge whatsoever of the other Gates, which seems to be proven true through multiple sources, including Psionic and Magical. O-Chul ultimately accepts that the prisoners will die, seeing that there is nothing he can do and seeing that the Twelve Gods are tasking him with enduring. Redcloak, now certain that O-Chul knows nothing, commands his hobgoblins to release the prisoners back to their cells, where they tell stories of his heroic stubbornness and will. (This despite Redcloak's attempt to spin the situation as O-Chul uncaringly allowing their deaths, while Redcloak spared them.)
O-Chul was returned to his own cage, next to the box of the very Monster in the Darkness. It was there revealed that the two have been regarding each other on a friendly level, and have been sharing their daily food with each other as well as playing games with each other (O-Chul has mentioned teaching the Monster how to play Go). The Monster still referred to O-Chul as "Mr. Stiffly" despite O-Chul's constant attempts to correct the Monster; O-Chul refers to him as "Monster-san."
Suddenly, Vaarsuvius teleported into the City to kill Xykon. Taking advantage of the surprise, he broke a bar from his cage and stabbed a Cockroach with it. He bid the Monster farewell, and only then did it call O-Chul by his name correctly. O-Chul charged against Redcloak, survived Disintegrate, stabbed his eye and removed his phylactery, but the Goblin was able to cast Word of Recall before being killed. O-Chul killed Jirix with the bar and in vain tried to destroy the phylactery that would kill Xykon; he withstood his Meteor Swarm but not the Maximized Lighting Bolt.
Meanwhile Xykon withdrew his attention from V who now became invisible. V tried to escape but changed the mind, found healing potions from Jirix' body and helped O-Chul to regain his health and once more grasped Xykon's phylactery while trying to strangle V. The two started to run away from the lich and V sent Blackwing to throw the phylactery in the rift.
Having no hope of escape, O-Chul charged against Xykon, who responded with a Mass Hold Person. But before dealing with them, he flew behind Blackwing to retrieve the amulet. O-Chul managed to break free from the spell, but Xykon appeared once more, angry for losing the amulet. Preparing to cast Meteor Swarm inside V's and O-Chul's mouths, the Monster begged them to escape, teleporting both V and O-Chul above Lien and Hinjo.
Upon his return, O-Chul debriefed Hinjo with the intelligence he collected while captive, including a near complete list of Xykon's spells, which he gave to Roy. He also thanked Belkar for abandoning him, due to the advantages the group gained from this act. (However, O-Chul also threatened to kill Belkar if he ever treated anyone else in this manner.) He then departed with Lien to check in on Kraagor's Gate (with the destruction of Soon's Gate, the Sapphire Guard was free of their oath not to look in on the other Gates). Since O-Chul has been seen briefly on an small island during a period of rain (which helps O-Chul sleep).
Since he has departed Azure City, O-Chul's efforts to encourage the Monster to think for itself are beginning to show themselves; as the monster is seen endeavoring to discover what became of O-Chul and hoping for his safety.
With the assault of Varsuvious on team evil, O-Chul was able to effect his escape with the unwitting help of The Monster in the Darkness.