King Enma Jr., usually addressed as Koenma (コエンマ, "little Enma"), is the son of Enma (rendered as "Enma" in the English manga and "Yama," the Sanskrit name, in the English anime), the ruler of the spirit realm, and the one responsible for appointing Yusuke Urameshi as spirit detective. His Japanese seiyū is Mayumi Tanaka and his English voice is Sean Teague in the Funimation dub. In the first movie, The Golden Seal, he is voiced by Brianne Siddall. In the second movie, Bonds of Fire, he is voiced by Eric Stuart. Koenma's name is a combination of his father's name (Enma) and Ko, Japanese for "child".

Koenma first appears as a toddler, which he is often teased about, though he is at least 700 years old and has extensive knowledge of the universe. Koenma is a basic comic relief character and is often shown cowardly. However, if Yusuke or any of the others are in immediate danger, he will help all he can. He comes off as a good leader and a nice guy for the most part. His job in Spirit World is to decide who goes to Heaven, and who goes to Hell. Some think this should be his father's job, but King Enma is too busy on business trips most of the time. Koenma can turn into his teenage form, after thinking that Yusuke and Kuwabara will make fun of him. Also, he has a Jr. sign on his forehead, and still has his pacifier, which has been carried in his mouth for most of his existence, storing up his spirit energy. When the power is unleashed, it creates the Mafukkan, Spirit World's most powerful defensive spell. Depending on how much energy Koenma places in the pacifier, he can seal demons of varying strength. At the time of the Chapter Black Saga, Koenma can seal an S class apparition with ease.

He is responsible for the Spirit Detectives, which is a job he requested. When Shinobu Sensui attempts to unite the human and demon worlds, he feels responsible for him. In fact, when he unleashes his Mafukkan on Sensui, he is prepared to suffer with him, unmoving for eternity, as atonement. Koenma betrays his father when he sides with Yusuke after the Spirit Defense Force wanted to kill Yusuke for being a Mazoku. So Koenma lives in the human world for a short while, afraid of Enma's fury; Koenma later returns to the spirit world. In the manga, Koenma overthrows Enma after Enma was discovered to have deliberately allowed demons into the human world to cause conflict.