Koenma, the son of the ruler of the Spirit World, hence the name, Enma Jr (Though King Enma is referred to as King Yama in the english dub of the anime) is the boss of Yusuke Urameshi, the spirit detective. As a prince, Koenma has many subordinates, especially his ogre,  Jorge Saotome. He is rather bossy, as is required of someone of his position, and we see this especially with his servant, and with Yusuke. Though, Yusuke doesn't really take his orders well, more often than not, Yusuke will start an argument with Koenma regardless of the topic, showing to have a friendly rivalry with him.

Koenma, despite his appearance, is also in fact, not a child. He is estimated to be older than 700 years old, which highly contrasts with what his initial appearance gives us. 

Koenma has a rather nasal and high pitched voice, and voice actor's natural voice sounds almost identical to it, so you would have to have a quite a high range to begin with. He has a common North American accent, but sounds a little bit strange at times, this may be because Sean Teague is a Texan. Keeping this in mind, don't force a Texan accent, because Koenma still doesn't have a Texan accent, just strange pronunciation at times.

For reference:

https://www.youtube.com/watch?v=0PtffA6-t54