He has two forms: the cute toddler form and the kinda cute teenage boy form (he still has that pacifier in both forms) and he is the son of Enma Daioh, the ruler of Rekkai (the spirit world).  When Yusuke first saw him, Yusuke laughed his head off at the sight of a tiny toddler sitting behind an office desk   Anyway, this "little toddler" is actually 700 years old, and he's in charge of the entire Rekkai when his dad's away (which is pretty much all the time).

Koenma is a bit egotistical and bossy sometimes (couldn't blame him, he is the boss!).  Still, he risked his neck for Yusuke quite a few times, and he has a lot of power behind that pacifier of his.  He scolds Botan constantly and beats up poor Saotome George a lot, too.

Seiyuu:I dunno... sowwy but if you do, tell me! (I know it's a lady, though)