Kim Bok-joo: Starring Lee Sung-kyung

Having a dad who was a former weightlifter, Kim Bok-joo is blessed with powerful muscles. She is currently a college sophomore at a athletics program where she is a weightlifter with a promising career ahead of her. She is very close to her dad who is ailing from chronic renal failure and needs blood dialysis. Because she has a muscular build, she was never asked out on a date by a boy. But one day, Bok-joo has a fateful encounter with a man and is smitten with love. After finding out that he is a weight loss clinic physician, she signed up for an 8-week weight loss regiment at his clinic. She shockingly finds out that he is the older cousin of her on-campus nemesis Joon-hyung.

Jung Joon-hyung: Starring Nam Joo-hyuk

He is a talented swimmer who thrives in the pool. In his first major swimming meet at the age of 17, he was disqualified for a false start. Ever since that disqualification, he has a paranoia about false starts. He has high standards about which type of girl he’ll date and only likes extremely pretty girls. One day, he is reunited with his elementary school friend Bok-joo who once saved his life long time ago and learns that she is in the weightlifting program at his college. After finding out that Bok-joo signed up for a weight-loss program at his cousin's clinic because she has a crush on him, Joon-hyung can't help himself but to make fun of her.

Song Shi-ho: Starring Kyung Soo-jin

She started rhythmic gymnastics at the age of 5 and after winning the silver medal in the Asian Games at the age of 18, she continued on a winning streak in all of her domestic gymnastic events. But a second growth spurt restrains her from performing in the best condition. The psychological stress of competing at the highest level led her to make a critical mistake and gets kicked out of the national team training camp. Her ex-boyfriend Joon-hyung is the only person that Shi-ho can be totally open about her thoughts and feelings. She finds it unsettling that she craves Joon-hyung’s companionship because she broke up with him fearing that she might be getting herself into a dependent relationship. However, Shi-ho still longs for Joon-hyung even though he hangs out with another girl. And the girl happens to be her weightlifting roommate, Bok-joo.

Jung Jae-ee: Starring Lee Jae-yoon

He is Joon-hyung’s cousin. He swam in swim meets when he was younger but an injury forced him to stop swimming. He then focused on his studies and got into medical school. Though he carries a strong sense of responsibility as a physician, he hates the sight of blood so he opted to become a general physician. As a doctor, his job is to listen to his patients but one day a special patient enters his life. He treats her more like a sister than a patient. Then one day, she confesses her feelings to him and he soon finds out that she is a weightlifter and a former elementary school classmate with his cousin, Joon-hyung. He finds it awkward when he goes out on dates with her because he gets a funny feeling that Joon-hyung is watching.