Yoojin is described by the groups PD as , "A person of few words ,but can also be very frank when things are need to be said. She ice cold and stand-of-ish on the outside, but has a sensitive heart on the inside . A certified trouble maker, that loves to pick on others as a means of fun (and friendship). She is very stubborn , short tempered , sarcastic , wity and tactfull. Occasionally arrogant and haughty - especially around people she is not particularly fond off. Its these traits that make her stand out from the group, its more of an asset really than a liability... it makes people ...look forward to what she will do next. She is very much like Kim Heechul of Super Junior, it makes us wonder why they aren't siblings!".  He said this as he was being interview regarding SM's new girl group.

At first you might mistake her for a person with a ice cold personality because of her headstrong and frank exterior toped with her stoic expressions and grace. She is the type that is reserved, yet assertive when the situation calls for it like when she is faced with gossip and rumors about her. In these times she is not someone who would just "Let it be" , instead she would be on a full confrontation with those who talk "" about her. She is the type to follow her own beliefs and stand her ground to protect it. Yoojin is also the kind of person that uses informal language towards people, even her "oppa's" and "unnies", but it is not because she is rude or anything. It is because she see's them as a friend , someone who is her equal. For her it is a way to easily get close to others in her own little way. Another thing about her is her love for mischief - but this only comes out when she is with her close friends.

She often drifts into her own "world" like staring blankly into space and does not notice others around her. Some say it is because she is thinking of ways to Prank or Blackmail people. Sometimes this is true but in in other times she is only thinking about random things like or drifting of into dreamland. Yoojin is also painfully honest at times, she is not the type that 'sugar coat' her words. She's the girl who is not afraid to voice her opinions and beliefs nomater how offending rude it seems at times. This is because Yoojin is the kind of girl that  would tell you the truth as it is, even the once that you do not wish to hear. She does this because she has a strong sense of honesty, and a firm grip on what she believes in.  As she once said,  " The truth no mater how painful and unpleasant, is the only true reality and we are all entitled to know our own truths." Others tend to missunderstand her brutal honesty and would often think badly of her because of it. She is even known as “ Sniper Jean”  because of her words often hit pople in the bullseye. The amazing ability to push the buttons of people with just words.

Though she may seem cold as ice on the surface , underneath all that is a girl looking for affection and friendship. Yoojin is actually a sentient soul that wish to be understand. She may tend to close of her doors from others, but when she does open them you will see the fragile girl she is inside. She just tend to distance herself to others because she grew up surrounded by people who only befriends her because she's one of the "populars" - she was popular because of her "pretty face". It is because of this she learned not to trust people easily and would often put a wall around her heart to keep people away. As they say the people with the hard exteriors are actually the most vulnerable inside. This is because they tend to hide their insecurities by being distant and harsh towards others.

Yoojin is a person of great wit , tact and knowledge . However, in matters of the heart she is a complete idiot. Often times other people around her are the first to notice the change in her action. If she were to know her true feeling, she is sure to deny them and cover them up by being rude and quarreling with the one she likes as hide her embarrassment. Only those with keen eyes are able to see through her facade (Her members & Heechul) . It is one of the rare opportunities that people are given the chance to see a different personality of hers. In these occasions they see her as flustered and in denial of her true feelings, and would often love to about it by saying thing like "awww.... are you blushing?" or "has the ice princess finally fell in love?". Another thing that Yoojin is once she has set her mind into doing something she is sure to strive for it until she attains her goals. As she had said once “quitting is not an option.”

Her mother died while giver birth to her, thus she was only left with a father who was never really present in her life  and an older sister (Subin) who was always being compared to her. The death of her mother caused a great effect and depression to her father. He was often busy with work and seldom be home, when does come home he would lock himself in his office immerse himself in drinking alcohol and doing more work. Often he would ignore his younger daughter, but when he does it is often to giver her a scolding for not being as smart, as good and everything that is her "perfect sister". Being constantly compared to her older sister caused a feeling of inferiority in young heart and mind Yoojin.

It was at the age of seven that her father decided to have Yoojin study  abroad, under the care of her uncle  ( her deceased mothers younger brother). He did this  thinking that it was for the best since he could not give the care that Yoojin would need, especially since he is still struggling with the death of his wife , even after many years.  At that time Yoojin's young mind did not understood his actions and thought he was sending her away be cause she was not like her "perfect sister" and because he hates her for causing the death of his beloved wife. Through the years Yoojin grew distant to his father even so she still still longed for his affection and love. Yoojin would often try to gain his favor by working hard on every thing she do and strived to be like her "perfect" sister Subin. This is why when she was presented with a chance of becoming a trainee at one of Korea's top entertainment company she did not let it pass and grabbed the chance. She migrated back to Korea to pursue her training and studies their It was then she promised herself that she would not be a disappointment to him, and that one day her father will be able to show her the love and care that she has been craving for so long.

Life in Korea was not an easy one, it took long for her to adjust to the different environment. Having stayed so long in the US, she  felt alienated to her own homeland. It was hard to her to make friends, for she could no longer speak her native language. Often people misunderstood her actions and thought of her as rude for it. This  lead her to be an outcast in her own country, a feeling she tried to cover up by being cold and indifferent to everything around her. She then started to build up walls to protect her self, even so she still longed for the company of a friend. This she found in the person of Kim Heechul, who was never quick to judge a person yet is very vocal and true about his honest opinions.  Though, he is her senior by many years Heechul still treated Yoojin as a friend who is an equal at times even as a sibling. This is probably because Heechul saw in Yoojin the same situation that one of his friends (Hankyung) was in during their trainee years.

How she became a Trainee:

When Yoojin was 13 years old (and a half) , Global Auditions was being held at their city. At that time auditioning at a Korean entertainment company was not in her agenda. She only went to the auditions to accompany her friend who is an avid Kpop Fan. As she was waiting for her friend to finish one of the Judges took notice of her "looks". Her friend was not lucky enough to be one of those who passed. However, it was different for Yoojin for after the auditions one of the Judge approach her asking if she wanted to become a trainee at SME and gave her a calling card. She was hesitant at first but after a few days of giving it tought,  she decided to give it a shot and call the talent agent. 

Yoojin was then invited to come to Korea to sign the contract and pursue training.  She was initially trained to become an actress and model. However, upon noticing her great potential in the field of dance and music - it was decided that she be  trained to become an Idol. This was all done without the knowledge of her father, at that time she was not yet ready to let him know fearing that he would not take this news well. It was only on the day that SM announced to the public  that Yoojin will be a part of the new girl group,  did his father found out.

• Heechul & Yoojins meeting: At that time Yoojin was still a new trainee, she has just started her Training in Korea. She was having a hard time adjusting to her new life and the customs in Korea - Because of this she was outcast by her peers and fellow trainees. On one occasion the other trainees decided to pull on a mean prank on her. They took her things from the locker rooms and shredded her clothes, ruined her shoes then threw it at the trash bin. However, Yoojin saw them on the act of doing so and decided that "this is the last straw" and marched up to them. Yoojin then gave them a "piece of her mind" , tough shocked and a bit worried they still tried to pull the "tough act .One of them (the leader Jaemin) said,  "tsk! You deserved it...tought you were better than us just because you came from the states and have a pretty face?! tsk!  Well lets see if you will stay pretty if i cut up your face too!".  At that time Heechul was passing by the halls after his meeting with Super Juniors PD, he heard the commotion going on and decided to check it out. He made it just in time to see a group of girls ganging up on one girl, seeing that one of them was about to hit the "helpless" girl on the face he acted on instinct and stopped the raised hand.  Pale faced and wide eyed with shock they all stared at the "new arrival", it was then that Heechul decided to speak with a threatening tone that seemed out of place in his  feminine face.  "What do you think your doing? You do know that bullying is against company rules and can get you kicked out, do you?", he said. Trembling and pale with fear  they hastily said their apology to him and ran with their tails between their legs. And that was the beginning of Yoojin and Heechuls close friendship, since then no one has ever tried to pick on her, even the girls who once bullied her have tried to get on her good side. However Yoojin is not quick to fool , as she says "I can forgive but I wont forget".
• Heechul being interviewed about the upcoming Girl group, and him talking about his close relationship with Yoojin. He also talks about how she can seem closed off and cold at first glance but is actually really  dorky when you get to know her.
• Yoojin and Xiumin being teased about their relationship that is like children who are having a crush on each other but keeps denying it, much to their embarrassment.
• Yoojin being teased during a variety show about how she gets scared to sleep alone and "crawls" into her members beds after watching a horror movie. This is something she strongly and embarrassingly deny much to everyone's amusement.
• Girlfriend guesting on Strong Heart and Yoojin talks about her relationship with her family, and how she felt about both her sister and father. She also talks about her struggles when she first came to Korea the bullying and being outcast by her peers.
• This is happens a few days after the Strong Heart show on the groups Reality Show. The members of Girlfriend decided that it was time that Yoojin and her family patch things up, so they set up a plan for them - without Yoojin knowing. The plan was to have them talk about their issues. After school Yoojin receives a "mission card" from the stafs of their Reality TV Show. It instructs her to meet up up with her members  at a restaurant, she then follows the instructions  not knowing it was only a ploy for net to meet her family. Arriving at the place Yoojin sees her members sitting around at a big table she sits down and chats with them then after a few small talk they tell her that their  are someone whom they like her to meet. It was then her father and sister enters the sin , Yoojin was shocked to see them and looks at her members asking for an explanation. They then tell her that its time that she talks with her family and leaves them alone to talk. It started out with an awkward conversations of "how are you's", both sides hesitant and uncomfortable with the situation. That is until her father decided to  speak first saying that he is sorry. Her father confessed that he knows he have not been the best dad, and that he did not hate her and never did thought of her as the cause of her mothers death. As for her sister, she  confessed that she never really knew that Yoojin felt that way and that she too envied Yoojin.  In her Yoojin is the lucky one because she was free to pursue her dreams as she wished, but for Subin it is different because she have to live under pressure of having no flaws and the need to be perfect -for it is what people expect of her. With those said they were able to patch things up and the show ended with their tearful yet hart warming reunion.