Koenma Daioh is the only son of the Rei Kai god , Enma Daioh. Koenma usually shows up as a toddler form, but he's much older than anyone else. He transforms as a mature person when he comes to Ningenkai so that no one would make fun of him. The only exception when he transfroms as a mature person is his pacifier. His pacifier saves alot of energy for future use. During the Sensui Saga, Amanuma a.k.a. Gamemaster died from his loss in his own game / territory. Since Amanuma is still too young and unaware of Sensui's plans, Koenma saved him using his pacifier, using the stored power inside it. The power of the pacifier can also release a sacred shield that entraps any youkai/person in it, including Koenma himself. Koenma once used this to Sensui, but unfortunately it did not work out.
Koenma often does alot of paperwork in rei kai. He also has a loyal assistant named George Saotome ( his name was barely mentioned in the series )
Koenma stands as the head of Yusuke's team since he was the one who appointed Yusuke as a rei kai tantei. He gives all the necessary information to Yusuke about his missions. He sometimes help Yusuke's team if he had to. This paved a way for Koenma and Yusuke to be friends. One time during the Sensui saga, Enma Daioh ordered his troops to eliminate Yusuke because he carries a Class S Youkai blood which can bring chaos to Ningenkai when he transforms to a youkai. This transformation is possible after Yusuke dies. Enma believed that if Yusuke dies and transforms into a youkai, his soul will also become a demon. Koenma, after knowing this from his father's troops, he disobeyed his father for the very first time and helped Yusuke to survive ( I guess Yusuke didn't need any help anyway 'cause he was already powerful enough to save himself from Enma's Class A soldiers ).