F4 member Prince Song was part of the LG Twins' children baseball team when he was younger. Although he is part of the Invincible Baseball team, no one knew about this until pictures of his childhood were revealed. Along with the picture of him in a baseball uniform, it was revealed that little Kim Joon liked baseball so much that his parents even signed him up for the LG Twins. However, despite the experience he had as a child, Kim Joon fails to impress his teammates and has earned the nickname of "Bubble Joon" in the show. At first glance, he may seem like a mere pretty boy with no athletic skills, but it has been revealed that Kim Joon likes to play basketball and go boarding, and is in fact a sports mania! A representative of Kim Joon's agency stated, "Kim Joon has a lot of affection towards 'Invincible Baseball.' Even though he is not on top of his game at this point, we are content that he is developing better baseball skills with the new coach."