Casual Story is one out of the two modes available in Mystic Messenger, the other being Deep Story.
Through Casual Story, the player will be able to obtain the routes of Yoosung, Zen and Jaehee Kang depending on their responses in the chatrooms during the first four days of the game. Because 707 and Jumin Han are only romanceable in Deep Story, the player will not be able to get their route even if they get their hearts during the Day 1-4 time span of gameplay in Casual Story.
Casual Story: Bad Ending
If the player misses too many chats, or obtained only Jumin Han / 707's hearts, or did not obtain enough hearts of either one of the three to get a route by Day 5, the player will obtain a bad ending at the start of Day 5.
Prior to Day 5, Zen asks the player if they are having fun reading their conversations, and says he dreamt about the player being an imaginary person - which was rather convincing because they have neither seen their face nor heard their voice before. He begins to question if Seven is just tricking all of them and if he simply created an imaginary person under the player's alias to confuse them. Zen continues by saying that at the end of the dream, instead of the player, all of them are the robots saying programmed things instead. He eventually shakes it off, and both of them sign out for the night.
At the start of Day 5, Yoosung laughs at Zen's earlier messages about him joking that the members of RFA might just be robots. He mocks Zen in the chat for only realising now that they're robots', before going on to break the fourth wall and explain that the RFA members were originally humans, but Seven and V had indeed brainwashed them and are now programmed to act like robots. He says although it was nice meeting the player, they might leave even before the party starts since he thinks the player must find it boring talking to robots, and says that both of them have totally ruined V's plans.
Afterwards, Yoosung and Zen get into a heated argument about this matter, and Yoosung further explains that the entire messenger is an entire ruse created by Seven and V, and the rest of the RFA members are being used to satisfy until they're no longer needed and discarded; all for the sake of the RFA party. He says that Rika was so shocked by V's actions towards this matter to the point that she took her own life, and was told by her that 'V is using us like they're just machines', and that only Seven and V have free will, which explains why they are the only ones with classified information. Jaehee and Zen are worried about his mental state as a result, and decided to drop by his house. After both have left the chatroom, Yoosung mentions that 'V's scheming something evil and will do whatever he needs to carry out his plan', and believes he had only started to realise the truth after Rika's death.
In a following visual novel sequence, Yoosung opens the door to his home, expecting Jaehee and Zen, but is greeted by Unknown instead. Unknown says that given the current situation, Yoosung would be taken away first to paradise and claims that both humans and machines can be happy there. Believing he was sent by Rika, Yoosung agrees to follow Unknown and requests to be awakened from his 'brainwashed' state as he prefers to be where Rika is rather than RFA parties. Unknows replies that he knew he could convince Yoosung to follow him and invites him to travel to eternal paradise known as Mint Eye (Magenta).
Soon after, Jaehee then questions the player about Yoosung's whereabouts since he has gone missing, and notes that they should bring him to take a mental examination after he has been found. Jaehee proceeds to break the fourth wall by telling the player to please make up their mind before making any further decisions (since the player did not obtain a specific route over the course of the four days of gameplay).
Afterwards, the bad ending screen appears, and the player will be brought to the main menu screen.