Kim Hyun-joong
"One 4 U" and "Never Let Go" were written and produced by the multi-platinum Korean-American music producer, songwriter, and multi-instrumentalist Steven Lee. He also produced the song "So Cool" and made additional engineering on the song "To Me", of their labelmate Rainbow. He also produced songs of the group SS501 and its members Heo Young-saeng and Kim Hyun-joong.