Seho is a supporting character in The Coma: Cutting Class.
Seho is a secondary antagonist in The Coma. By mere accident, Seho was helping to clean out equipment from storage and happened to come across a recorder (The instrument). When he played this recorder, it would take him into the coma, where he struck a deal with a "Shade" and obtained control over her. After learning of this instrument's abilities, he begins to take lessons. There was even a point where his father noticed the recorder and was so happy for his new school activity, he pitched money in. However, the money was used for Seho's own personal greed.
Seho continued travelling in and out of The Coma for his own benefit and doing as he he desired. On the day of exams, he discovers that Youngho is in his realm, much to Youngho's disbelief whom mentions there being an evil version of their home room teacher, Ms. Song. Seho seems to be in disbelief of everything Youngho says. Once Youngho mentions that they should call the police, Seho states he forgot his phone upstairs and goes to retrieve it.
Later, Seho can be found in the top floor, school bathroom of the North Building. He states that he now believes Youngho about seeing a demon version of their teacher, Ms. Song and decides to remain in the bathroom for safety. After a while, Seho is found in the school basement searching for an object and pushing Youngho away from him. It is then that an evil, female spirit decides to possess him, wrapping herself around Seho's head. The spirit strikes a deal with Youngho that if he can find the object that Seho lost, they can both return to their realm safely. However, once Youngho gives the missing object, a recorder (the instrument) that is also a relic; the spirit attempts to attack Youngho, much to her failure (Thanks to Youngo's relic that belongs to Ms. Song) and disappears with a still possessed Seho.
If you get the true ending, it's discovered by Yaesol that Seho had been switching his grades with Youngho, due to Seho's actual grades being the ones slipping and Youngho being one of the head of his class. After the events of the first game, Seho decides to keep an eye on Mina while most likely currently possessed by "Vicious Sister".
UNDER THE BLOOD MOON
"Seho" continues to stalk Mina, even observing as she tries to dodge rejecting Myung-Gil Kang as Dahy-yun (Mina's best friend) has to deal with him to her displeasure. Myung-Gil then notices Seho, referring to him as "Sehoser" similar to how he also calls Youngho, "Younghoser". Seho leaves and later on in the day, enters a room while being observed and then followed by Mina Park. After tricking Mina Park to follow her into the room, he sets up a trap and steals the relic (Ms. Song's pendant that Youngho was holding) from her. He retreated after the "Ghost Vigilantes" came close to catching up with him and dropped his lighter in the process.
Mina eventually runs into Seho outside of the school on the way to the Police building. Seho acts surprised to see Mina and feigns ignorance yet again, saying he'll follow her. As she enters the police building, he barricades her inside for "her own good" and more likely to keep her out of trouble. It's assumed that Seho spends most of his time helping Vicious Sister out as much as he can.
The next time Mina and Seho meet is at the hospital. After a trap is set, knocking out both her and Father Minsu, Seho ties them up. As Mina and Minsu wake up, Seho mentions how him and Youngho use to be boyscouts. He then tells Mina that the "so called ghost vigilantes" are here to kill Youngho in order to stop him from being Vicious Sister's catalyst into entering the "Waking World" (The normal realm). He then proceeds to state that Mina is a thorn in his side and how she's made things hard on him, especially helping Yaesol who's "like a friggen' ninja". After, he explains that Youngho has a very rare skill where he can withstand being in the Shadow Realm for more than one day while most who stay longer than that, go into complete madness. Thus making him a prime candidate for a full possession by Vicious Sister. He notices that Mina found his zippo lighter and decides to let her keep it because he's "such a nice guy". Seho then returns to Vicious Sister to help prepare for the ritual.
After a while, Mina appears alongside Yaesol as both Seho and Vicious Sister have Youngho trapped in a huge, plant-like pod. Seho then makes his intentions clear as he asks Vicious Sister that after all this, she would let go of possessing himself and allow him to return to the Waking World. Yaesol tells Mina to help destroy the core of the plant while fighting off Seho and Vicious Sister in physical combat.
Once Vicious Sister is defeated, regardless of which ending you get, it's said that Seho hasn't been to class in a while. Thus it's assume that he's either dead or more likely, trapped in the Shadow Realm forever. His fate is never made completely clear, but it's safe to assume that either one is accurate. In the first game, it is also stated that over time, you lose memory of your name thus completely destroying your sanity even as a Shade. Thus, his fate might not be much different.
Seho seems like your average, ordinary nerd. However, there is more to him than meets the eye. Despite his friendly demeanor, he can be quite deceptive, underhanded and selfish. He tends only to care for his own safety despite what he might say and is willing to throw his friends under the bus for his own gain. Overall, Seho isn't a very trustworthy person and becomes less so as the series progresses.
-During the fight sequence against Yaesol, Seho's arm outstretches very similarly to a scene in the famous anime, "Akira". Whether this is some ability he gains from The Shadow Realm or just a fun reference is made unclear.
-Many notes written throughout the school are Seho's. It mentioned how he found the artifact, how he's been entering this realm and been attempting to even rule over it.