Jae Kook (Jang Ki Yong) is the brother of Young Eun's (Song Hye Kyo) ex-boyfriend in Now, We Are Breaking Up. For the longest time, she had believed that her ex-boyfriend had dumped her because he did not meet her as planned when she returned from Paris to Seoul.
However, it was Jae Kook who informed her that his brother, that is, her ex-boyfriend, had met with an accident on his way to meet her. He succumbed to his injuries which led to his death in Now, We Are Breaking Up.
Of course, Young Eun was shocked. She also grieved for the man that she was unable to forget. She also realized, however, that she was attracted to Jae Kook. This was not just sexual, but something about the photographer comforted her and yet excited her. She refused to let her attraction grow, though. She made a conscious decision to stay away from Jae Kook in Now, We Are Breaking Up. He persisted.
It was his comforting presence in her vicinity that broke down her resolve. The more she interacted with him, the harder she fell for him. That's how the two had decided to give their relationship a chance. If they had to break up, Jae Kook said that they might as well take it all the way. Hence the title "Now, We Are Breaking Up."
Who opposes Young Eun and Jae Kook's relationship in Now, We Are Breaking Up?
Young Eun's ex in Now, We Are Breaking Up, Soo Wan, was actually engaged to Shin Yoo Jung. She had seen Young Eun's video on her fiance's phone, and that was how she had recognized Young Eun after all these years. Before his death, Soo Wan and spoke of his interest in Young Eun to Yoo Jung. He had also informed her that he wanted to break their engagement. She begged him to reconsider his decision and hoped that he would come to his senses.
Her unresolved anger resulting from Soo Wan's death is now directed towards Young Eun in Now, We Are Breaking Up. This is the reason why she didn't want Jae Kook to date Young Eun. She tried her best to stop the two from getting more involved with each other. She first called Young Eun from Soo Wan's number. She then hinted about Jae Kook's relationship with his stepmother before he was ready to reveal her identity.
She continually claimed that she was worried about his mother. The truth was that she was angry with Young Eun for her relationship with Soo Wan in Now, We Are Breaking Up.
She also spoke to Young Eun and tried to intimidate her. It failed, and all this did was bring Young Eun closer to Jae Kook. Now, she learned that Soo Wan was a cheat. She was momentarily mad at him, but the revelation did not make any difference to her. She and Jae Kook realized that Soo Wan was no longer a part of Young Eun's present.
Jae Kook also realized that he must put an end to Yoo Jung's interfering ways in Now, We Are Breaking Up. So instead of encouraging her, he told his mother the truth about the woman he was interested in. Can he face the consequences of his confession in Now, We Are Breaking Up?