Dear Door

Kyungjoon Do: A police officer who when pursuing a suspect, recognizes a tattoo symbol linked to recent cult activities. Catching the escaping criminal, Kyungjoon affirms he is under arrest for unlawful assembly, fraud, and kidnapping. Knocked out and waking up to find himself tied up in a building on fire, Kyung-Joon struggles to escape. Maintaining his composure, Kyung-Joon thinks there has to be an opening somewhere. Identifying the same pattern on the interior of a building, he wonders if they were having some sort of ritual. Resolving to get out of here, Kyungjoon means to call for backup after. Passing out, he is saved by a demon named Cain in his unconscious state and becomes Cain's "door", a vessel that demons use to travel between the human realm and Hell. Arriving home, Kyung-Joon speaks about his day to Soyoung, his girlfriend who passed away. Tending to water the many potted plants he keeps in his apartment; Kyung-Joon hopes they will bloom beautifully. Kyungjoon has a very strong-willed personality and spirit, but also struggles with depression after the loss of Soyoung. He has been best friends with another police offer, Jinyoung, since they were young. An accomplished shooter, when firing at tendrils Kyung-Joon also uses his environment to his advantage in a fight. Supportive of those he cares about, whether human or demon, Kyungjoon will go to great lengths to ensure that they are okay if they appear troubled. Kyungjoon strives to solve a case involving a religious cult. Although resistant to being a door for Cain at first, as their relationship develops, he becomes more involved in his role.

Cain: An incredibly powerful demon who has lived for 50,000 years, Cain chooses Kyungjoon to become his "door" between the human realm and Hell. He often needs Kyungjoon's help to recharge his mana. Able to change into a winged demon form, his other traits include a real name that induces strange effects in humans, so he has Kyungjoon call him "Mr. Cain". Refusing orders to kill the police officer, instead Cain saves Kyungjoon. Reaching Kyungjoon and speaking if he understands what it took to enter the human realm, Cain requests for him to live and fulfill his role properly. Protective of Kyungjoon since he requires him changes to something more as Cain finds himself increasingly attached to the human. Cain does express jealousy, as well as an initiative to help Kyungjoon with the case at hand, but only for something in return. Despite his injury, Cain is still very intimidating and commands the respect of other demons. Even with his power diminished Cain is a very skilled fighter, yet also relies on others. Cain has a very laid-back and humorous personality and does not take anything too seriously, but will quickly lash out and protect Kyungjoon if he is in danger. While the human serves a convenient purpose to him in the beginning, Cain's feelings towards him develop into more. Cain also has two notable allies who have served him over the years, Sid and Ben. Sociable, Cain works quite well with those he knows.

Jinyoung Cha: A sergeant who has known Kyungjoon since they were young men. He is Kyungjoon's best friend and also a police officer in the same district. Praising Kyungjoon, he guesses their precinct is going to catch a big one soon thanks to what he found. Joking about it being ghosts or aliens, Jinyoung then jovially states for an officer to get back to work. Jinyoung is observed to be worried about every little thing, something he laughs off. Showing concern for Kyungjoon, he suggests they eat together having noticed he has been eating alone recently. Feeling he may be being pushy, Jinyoung says for Kyungjoon to call anytime he needs his best friend. His sister, Soyoung, was also engaged to Kyungjoon before she passed away.