Not much is known about Yoosung's mother, except that she works for the government, and rarely sees her son after he had moved out. She usually cleaned, cooked and took care of him like a mother should, when she had the time.
She was a "Home Economics" teacher for 24 years at public schools.  She is a licensed realtor, the president of the Hansorum Apartment community, and the director of the Phoenix Senior Citizens Center.  She has a certificate in cooking and multiple other fields, and is a member of multiple golf clubs and fitness clubs.  She also holds soap making classes. 
Her appearance is unknown at the moment. However, it's possible she has brown hair or purple eyes like her son.
In Seven's route, Yoosung's mother was only mentioned in few chat rooms. She visits Yoosung because Jumin called her and informed her concerning Yoosung's health and addiction to games, which possibly can ruin his health (sleeping pattern) and his studies.
Initially, Yoosung protested against his mother's stay, seeing as how Jumin did it without consulting him first. He mentioned that he was against regarding his mother's visit and temporary stay-in, but later on he admits that he actually misses and will miss his mother again after she leaves.
Yoosung found out that there was "a time that his mother was very sick", but she decided not to inform him about it. Yoosung revealed that he regrets not calling back his mother often, and felt guilty that he's not paying any visit to his parents just because he's enjoying his "unlimited" playing time of LOLOL, and has gained "independence" from his parents' (strict) guidance.
He is her son, and she took as much care of him as she could. Yoosung has mentioned that she's the one who sent him to college. He considers his mother to be the most diligent person on Earth. 
It is unknown what there exact relationship is, but in Seven's route he called her about Yoosung's unhealthy gaming habit.
- Yoosung mentions that his mother has a chronic (long time) illness; although Mrs. Kim claims to have recovered from said illness and to have been "alright" when she visits her son.
- Yoosung has mentioned that both of his parents are teachers.
- In South Korea, teachers are civil servants/government employees.