Ihyun Jo is in the band of criminals who attempt to take over the apartment building. He is one of few half-monsters. He is able to control his monsterization very precisely, as he has Dissociative Identity Disorder , and only his alter is actually affected by the monsterization.
Personality.
Ihyun Jo is shown to be very kind, offering one of his inhalers to Yuri Park, and explaining his wrongdoings. He seems to respect Joon, and will do as he asks. He is very empathetic to others and even monsters. His alter ego, however, has given into the monster. The alter ego displays sociopathic tendencies. It wants to be seen as an "anti hero," and is frequently maniacal and violent in its attacks. However, both sides appear loyal to Joon Shin.
Plot
Ihyun drives the bus that is used to kill the Steroid Monster. Later, as the criminals attempt to take over the apartment, he gives Yuri Park a new inhaler. Joon Shin attempts to have Ihyun choke Hyun Cha to coerce the latter to side with him, but Ihyun is forced to let go as Hyun refuses and Joon does not want to kill Hyun. When the Tentacle Monster attacks the lobby, Ihyun fights the Tentacle Monster. The Tentacle Monster wounds Ihyun, which causes the insane, monster-controlled side of Ihyun to take over. He complies with Joon's request to avoid unnecessary collateral damage, and fights off the monster long enough to douse and incinerate it.
The insane half of Ihyun remains in control. After Hyun agrees to work with the criminals, Ihyun and Hyuk scour the apartment for any monsters to kill. The two get into a philosophical disagreement over treatment of monsters. Ihyun believes anyone showing any sign of monsterfication should be killed on the spot, while Hyun wants to give the monsters the benefit of a doubt in an effort to rescue their humanity. Ihyun suspects Hyun is hiding something and enters his subconscious. Ihyun learns through Hyun's inner monster about the Infant Monster and goes to find it. Hyun chases after, but both find a cocoon in place of the monster. Ihyun attempts to destroy it with his monster powers, but is shocked when he loses control of his arm, which first froze and then attacked Ihyun. Hyun uses this moment to restrain Ihyun. At this point, Jisu enters, worried about Hyun. As the two debate over how to deal with the egg, Ihyun breaks free of the restraints and incinerates the egg. This unwittingly attracts a horde of monsters, but the maniacal Ihyun gets excited at the idea of fighting off many monsters, and abandons Jisu and Hyun to fight for themselves.
As he chases a monster up to the roof, he notices a Mysterious Woman standing at the ledge. This visibly shocks Ihyun, which a monster then uses to ambush him. Ihyun notices it in time and manages to beat it back and incinerate it. Ihyun contemplates what he just saw and wonders if he is hallucinating. He runs back to the roof when he notices a loud commotion and he witnesses Joon Shin being abducted by a monster in the woods and presumably killed. Ihyun then sees the horde of monsters approaching the apartment. Ihyun senses another monster approaching; however, because no one has encountered a flying monster before, he looks the floor and fails to locate it before it is able to get behind him. Wounded, Ihyun tries to fight the monster, but decides to retreat, realizing the monster has an extreme advantage in the open air. He is barely able to make it inside.