Koenma is the prince of Spirit world. His father is King Enma (or King Yama), who decides whether mortals go to hell or heaven. He is the one that gave Yusuke the ordeal to come back to life. Though Koenma normally assumes the form of a child, when he wants he can transform into a teenager, but he still keeps his pacifier which has stored spirtual powers.
Yu Yu Hakusho and all associated characters are copyright of Togashi Yoshihiro, Studio Pierrot, Fuji TV, Shounen Jump Weekly, and Shueisha. It's licensed by FUNimation Productions, Ltd. This is simply a fan site to it.