Mother Choi was the abusive, alcoholic mother of 707 and Unknown as well as the ex-lover/former mistress of the current Prime Minister of South Korea and a presidential candidate of the current election.
While she was alive, she an abusive and manipulative woman. Because the twin's father was a high-profile and married politician, she used the fact that she was his mistress and that she had children by him to her advantage to blackmail and to extort money from him, which she would mainly use for alcohol.
The twin's mother often used their father's position to strike fear in them and to keep the boys in line. She constantly told the boys that their father would send people to kidnap them and kill them, and if it wasn't for her, they would already be dead. Although the Prime Minister had attempted to kidnap them, it's never confirmed if he had wanted his sons dead; however, the twins consider her words to be true even after they had left their mother and had become adults.
Whether or not Mother Choi had genuinely loved her sons at some point in time or had only raised them into adolescents for blackmail material is unknown. However, she was hysterical when Saeyoung had disappeared to work for the intelligence agency.
She never physically appears in the game, but she likely has red hair and/or gold eyes like her two sons.
When he and Saeran still lived with her, Mother Choi would often send Saeyoung to run errands such as buying medicine, groceries, and alcohol for her. Like with his younger brother, Saeyoung was also abused by her, often being beaten and starved; however not as severely as Saeran was nor was he tied up.
Mother Choi abused him more because he was weaker and more sickly than his twin, Saeyoung. His punishments included having one of his legs tied to some part of the kitchen to prevent him from leaving as well as being beaten and being starved and dehydrated for days.
- Mother Choi's death remains a progressing mystery to be solved throughout the course of Mystic Messenger.
- In Saeyoung's Route, the player and Saeyoung are told she committed suicide 2 years and 6 months before the Deep Routes.
- In Yoosung's Route, it was hinted at by Jaehee that she was actually killed. Players are led to assume it was by Saeran.
- However, in the Mint Eye Believer's package that players can purchase on the Cheritz store, Saeran's diary reveals that he had recently escaped the home before she died. Though it appears to potentially be a murder, investigators cannot find out who did it and chalk it up to a suicide. Saeran is seen mourning her death despite the abuse he endured from her.
- In the Good Ending of Ray's Route, it is revealed after the credits that Mother Choi was in-fact killed by Rika in self defense. Upon attacking Rika for taking Saeran away, thinking Rika is one of the Prime Minister's cronies, Rika defends herself and accidentally kills Mother Choi in the process.
- In South Korea, the Prime Minister and the President are two separate positions. The President is the head of state while the Prime Minister is appointed by the President and acts as a Vice President of sorts.
- Cheritz revealed during the Otakon 2018 panel that Mother Choi's alcoholism is the reason Saeyoung refuses to drink throughout the course of Mystic Messenger. When Saeyoung drinks champagne at the end of V's Good Ending route, it is due to the stress he's endured by the course of the route that changed his mind.