If you haven't played Mystic Messenger completely, you might read something that will reveal to you the complete story of the game. So please pay attention!
I'm so glad we're hosting another party! I'm going to do better than last time so that Rika's not disappointed from up there.
—Yoosung, Comment from RFA Orientation Guidebook
Yoosung is one of the Casual Story characters available in Mystic Messenger. Yoosung is a sophomore college student who plays a game called LOLOL and invests too much time into it. Because of that, he does not spend as much time studying for college. Yoosung is also the cousin of Rika and cannot fully bring himself to completely believe that she is not alive anymore.
Yoosung is a really nice and lovable person, and is described by the other RFA members as 'a real puppy'. Yoosung cares about others a lot and loves to spend time with others. He is naive and believes things that are told to him more often than not, as shown during several of his interactions with 707. As a result, he is often unforgiving to his naive nature.
Because of the sadness in his heart from Rika's death, he often talks with the others about her and compares her with the player, even though he is fully aware that the player and Rika are not the same person (not counting Bad Relationship Ending 1 where he confuses the player as Rika). Yoosung is also often jealous of other people like Zen for his good looks or Jumin Han for his rich and different life.
He is also possibly known to get very possessive, as you go along in his route, during one of his calls he clearly states "I don't want you to look at anyone but me", possibly signalling that he wants the player to see only him as her significant other.
He is the shortest out of all the male characters, at 171 cm (5'7.5") and is also the most youthful appearance and age-wise. He has messy, bleached blond hair that he usually keeps out of his eyes with two dark brown hair clips that form an 'X' shape. He has soft violet eyes and a young aura to him in all his photos. His style of clothing usually contains something blue.
Before Rika's death, he had well kept brown hair and wore more formal clothing in most of his past pictures.
Despite how much he admired her, Yoosung hadn't actually met Rika until he was in 6th grade. As they spent more time together, he bore witness to the joy that came from the parties that the RFA threw and began to think of Rika as a magician of sorts. The death of Rika's pet dog, Sally, was the reason Yoosung chose to major in medicine to become a veterinarian. He did his best to get the top scores in all his classes. She shaped his whole life and when she died, he was thrown into depression. After her death, he hardly ever attended any of his college classes, so Seven decided to distract him by introducing him to the world of LOLOL, now his favorite MMORPG to distract him and help cope with his sadness.
As the player gets closer to Yoosung, Yoosung begins to take small steps into changing his life by quitting LOLOL. After confiding to the player numerous times about Rika, he begins to see some similarities in her and, at some point, begins to confuse her for Rika, to the dismay of the other members of RFA.
On the 7th Day, Seven reveals that he has found traces of the hacker in the previous three days, and that the hacker had modified the algorithm in the security system of Rika's apartment so that any RFA member who enters or exits the apartment will cause the bomb to detonate. Yoosung immediately becomes alarmed and quickly loses trust in RFA for putting the life of the player in danger, even after Seven fixes the security system. Angry, Yoosung pins the blame on V, mainly frustrated that V was able to move on quickly after Rika's death.
On the night of the 8th Day, V, who was called in by the other RFA members during the bomb incident, quickly rushes over to Yoosung's house to calm him down, confessing that he had hid secrets about Rika in order to protect him. He tells him that Rika had been suffering and is mentally unstable. Realizing how much RFA cares about him, Yoosung vows to become stronger to protect his loved ones as well.
Within the next following days, the threat from the hacker gets worse. Zen is harassed by a stalker outside of his house, and Jaehee receives a suspicious email from Mint Eye. Seven traces the e-mail back to its original location, and decides to go investigate. Wanting to protect the player, Yoosung volunteers to come as well.
At Mint Eye's base, Unknown catches Seven and Yoosung, threatening to press a manual switch that will detonate the bomb in Rika's apartment and the base itself. Yoosung grabs onto him while Seven escapes with the switch. By the time both of them leave the mountains, Yoosung is severely injured, but makes Seven promise not to tell anyone.
Yoosung is quite close with Seven. He often falls for the jokes/pranks Seven plays on him and is incredibly gullible. Seven was unable to make it to Yoosung's high school graduation due to being abroad at the time.
Yoosung distrusts V and even goes as far as to hate him. Yoosung thinks that V changed after Rika's death and is upset that V couldn't give more evidence about her death, but in the end 'forgave' him after moving on with Rika's death.
Yoosung is close with Zen, and he is jealous of Zen's good looks. Zen acts like an older brother towards Yoosung. Zen congratulates Yoosung on his relationship with the player, but warns Yoosung not to compare Rika to the player.
Yoosung is close with Jaehee, and Jaehee watches out for him and nags him to get his life together frequently. Jaehee watches out for Yoosung's health and his gaming addiction. Yoosung brings up at one point that he sometimes sees Jaehee as an older sister looking out for him.
In the Christmas special, Yoosung and Jaehee work together for the C&R fundraiser. Jaehee says she is proud of Yoosung for working so hard.
Yoosung is jealous of Jumin's good looks and money and will often tell the player to stay away from him. Other than that, Jumin and Yoosung are good friends, and at some point Jumin hired him as an intern. Jumin also give Yoosung the kitten, as seen in the After Ending, whom he named Lisa.