His favorite game LOLOL stands for League of Loneliness of Life, this is a parody to an online PC game League of Legends (shortened as LOL).
His LOLOL character seems to be based on the League of Legends character Taric, most notably his 'Armour of the fifth age' character skin pre-redesign
He prefers playing with females, and claims they make it better in the guild.
He often plays LOLOL from 10 P.M. to 4 A.M.
Despite of his addiction to gaming, Yoosung is actually smart enough to get a scholarship at his school; to the point that Jumin personally offered him an internship at his company.
His major is veterinary science and he was the fastest to get his Ph.D as mentioned by himself in Yoosung's After Ending.
In Jaehee's After Ending, he becomes a temporary assistant for Jumin, he also his intern prior became a vet in his After Ending.
In one of the chattoons on Cheritz's dedicated page, Zen calls Yoosung YS.
The university that Yoosung attends, SKY National University, is a possible reference to the three most prestigious universities in South Korea (Seoul National University, Korea University and Yonsei University). These three universities are often commonly known as the SKY Universities.
He studied 10 years to get into the university.
He, like Seven, enjoys Honey Buddha Chips. He savors one bag of chips for weeks, because they're so rare.
The one who put the black star (★) behind Yoosung's username in the RFA Messenger is Seven.
In Zen's route, Yoosung states that his mother is a housewife, his father works for the government, and his sister wants to be a teacher. He describes them as a normal family.
This contradicts information in Yoosung and Seven's routes where he states both his parents are teachers, which in South Korea are government workers.
In one of his phone calls in Seven's route, Yoosung admitted that he has small crush on MC, but after realizing another member of the RFA loves them much more, he's looking forward to other interests.
According to the Orientation Guide Book:
His email is firstname.lastname@example.org
His special talent is defending the peace of LOLOL.
He likes instant ramen, spicy rice cakes, omurice, fried chicken, and pizza.
He dislikes ginseng-flavored candy and things such as black garlic.
His most used apps are his class timetable, gaming forum, 10Gag, and the RFA Messenger.
He desires a gaming mouse, and an autograph of the LOLOL programer.
His favorite quote is: "It's not that people have humor. Humor is what moves people."; same with Seven.
Although 707 loves to play pranks on him, he mentioned that he will not go overboard since he thinks Yoosung would be scary if he reaches his limit.
Seven also claimed in a phone call that Yoosung could have yandere tendencies if you start to get involved with him.
He would like to play LOLOL together with the person he likes, or sit and chat at a cafe together.
He thinks that if he ever got into a love triangle, he would not be able to give up the person if he truly likes them.
For a first date, he has no preference on where to go as long as he and his date can talk to each other.
He would like to give his date shoes so that they could match.
In women, the feature he looks for (and at) are their eyes.
He adopted the cat who was found by Jumin outside his house during the events prior to the party. In exchange, Jumin would convince V one more time to get his eye treated and sent security guards to Rika's apartment.
In his After Ending, the cat is revealed to be named Lisa, implying that the cat is female.
In Deep Story, during a call with Yoosung, he has mentioned about him being an M, which stands for masochist. This is also referenced many times by Yoosung himself (mostly in response to his friends' ribbing), who remains blissfully unaware of what it means. 
He can cook well but has trouble with making Chinese food. 
Yoosung loves romance movies and dislikes horror movies, seeing as they make him uncomfortable. 
He has once broken the fourth wall. In his status he said "Seven, I feel like there is a bug in this game." 
His shoe size is 240. (EU: 38, US: 6, UK:5)
He and Jaehee share the same shoe size. In East Asia, there is no difference between men and women's shoe sizes.
It's his 3rd time to attend the RFA party in the Original Story, according to Jumin. 
In an outgoing call, he mentions he has abs although he admits it's not photogenic like Zen. 
Yoosung's Bad Story Ending 1 is the longest of all bad endings. It consists of 5 chat rooms, 2 visual novels, 4 outgoing calls, and 3 incoming calls. It starts from 17:00 until 23:30.
In an outgoing call in Jaehee's route, Yoosung has stated that he liked the idea of being treated like a maid, ordered around, and insulted some times; implying he is a masochist. 
According to Jaehee, he has changed clubs 14 times already within a year.
In April Fools DLC he is the vice master of his LOLOL guild, which is also called Omelette.  While in Original Story the guild is named Blood Dragon.
Byungwook, Yoosung's best friend in high school, has joined a cult, and his behavior scared Yoosung. 
In Casual Story and Deep Story, Yoosung isn't aware of the fact that Rika was adopted and was not related to him by blood. In Another Story, Yoosung is fully aware of Rika's adoption and lack of blood relation.
He mentions in a call that he used to write poetry in middle school.
According to Saeran's Diary:
His behavioral pattern is Home - SKY University
His weaknesses are Rika and family.
According to Rika's Scrapbook:
Rika needs him in RFA despite him being busy attending school.
In an interview as the new elected class president in high school, he rehearsed a lot for the campaign and speech because he has a mild stage fright. He had lots of people who advised him and few of them are Zen and Rika, who helped the most, but he didn't mention their names.
Yoosung's belongings: flip phone, new student ID (he lost the first one), wallet, pencil case (he carries it because there's a cute character printed on it), class printouts, and notebook.