Kim Jae Seop, better known by his former stage name AJ, was the vocalist, dancer and composer of the boy group U-KISS. He was also member of the boy group Paran. He appeared in Grey and Verbal Jint's video for the song ''Pretty Brown''. He wrote many U-KISS' songs like: Words that hurt me (together with Dong Ho), Believe, I love you, Love of a Friend, Top That, Obsession and Tell me. He also wrote the rap part for U-KISS' song ''0330''.
He used to live in the United States, New York and went to Columbia University of United States in 2011.
Jae Seop joined Rooftop Company and now is solo singer under the stage name Siyoon.