Kang Baek-ho and Ham Yi-seul have been best friends since elementary school. Baek-ho has never acknowledged his attraction to her, unaware that Yi-seul has been in love with him for more than 20 years. Finally, at the wedding ceremony where Yi-seul is about to marry another man (Jin-won), Baek-ho regrets that he never confessed his love for the bride. A mysterious conductor suddenly appears and offers him a second chance to win Yi-seul's heart. Baek-ho travels back in time to various events of their lives hoping to change the outcome of their relationship. But changing the future is not as easy as it seems.
His father died when he was a child, and he grew up in Yi-seul's family because his mother was never home due to her job. In high school, he was a pitcher on the baseball team, from which he retired after an injury.
Baek-ho's childhood friend. She is a selfless girl who puts others first, and has a very motherly attitude towards Baek-ho. In high school, she was the class president, a member of the drama club and the manager of the baseball team. Her dream was to be a sports agent, but she eventually became the PR of a baseball team.
Baek-ho's best friend, he is calm, rational and loyal. In high school, he was a pitcher on the baseball team along with Baek-ho and Tae-nam, but he left to pursue his true passion, cinema. He dreamed of becoming a film director, but he eventually found work as a cameraman.
Park Young-seo as Joo Tae-nam
Baek-ho's friend. He has a huge crush on Chae-ri from the sixth grade and is very jealous of any man who comes close to her, even if she doesn't reciprocate his feelings and often teases him for his short stature. Thanks to his interrelational ability and business acumen, he later becomes Chae-ri's manager.
A former baseball hitter, he was discovered on the first year of university by Major League, but had to give up the sport career due to an injury. He has studied at Harvard, where he earned a master's degree in business administration. He later returned to Korea and was chosen as a baseball coach by his former school, Haneul High School. He fell in love with Yi-seul, and later becomes her fiancé. He is a well-educated man from a prestigious family, and the founder of Sport K.
Lee Doo-il as Jo Kook-dae
The owner of the tavern where Baek-ho and friends spend their free time. He is a jovial and cultured man who speaks in half-English.
Kook-dae's niece, who lives with him and works as a waitress in his tavern. She is the only survivor of a fatal accident that killed both her parents and since then, she hasn't spoken. Because of her closed and creepy attitude, she is avoided by everyone. The only person among Baek-ho's friends to pay her attention than usual is Chan-wook; therefore, she has a crush on him, and understands what he wants even before he has finished speaking. She is good at singing and playing the guitar.
Yi-seul's mother. A loving woman who gives many tips to Baek-ho and considers him a son.
Go In-beom as Oh Tae-beom
Yi-seul's maternal grandfather. A bossy and stubborn man used to get what he wants. He was a great baseball coach feared for his severity. He loves Yi-seul more than his own daughter, and sees Baek-ho as the son and grandson he has never had, teaching him all the secrets of baseball. He died while Yi-seul was attending her first year of college.