Koenma normally appears as a toddler with a red obi sash tied at the waist of a pink-trimmed blue robe over brown pants and black shoes. In his first appearance in the manga, Koenma was seen wearing a bib on his robe, though this was never seen again in the later chapters, nor in the anime.
However, when venturing into the human and demon worlds, he transforms into a handsome teenager with medium length brown hair, parted down the middle (though he occasionally appears in his true toddler form as well).
In both forms, though, he retains a similar outfit, as well as the "Jr." mark on his forehead and a blue pacifier kept in his mouth. He appears remarkably young for his age (exactly 670 years old in the manga, 666 in the beginning of the series), being that he has existed for several centuries prior to commissioning Yusuke.
In the Three Kings Saga, Koenma wears casual clothing as opposed to his primary outfit due to disobeying his father, he wears an outfit similar to Shizuru's as the orange long sleeve shirt is replaced with a button shirt and the "Jr." mark on his forehead was covered in a purple headband.
Later, he wears a costume similar to Tuxedo Mask's and his headband is red.
Being a prince, Koenma has many subordinates, especially the ogre Jorge Saotome. He is (mostly in his toddler form) normally abusive, with a rather bossy attitude and also a loud prince, commanding those around him to accomplish his tasks, whether they work for him or not. He gets along with Botan, however he has a sort of sibling rivalry with Yusuke.
In the original Japanese version, he addresses himself as "washi", a term that elderly Japanese men and women use when talking about themselves. This may either refer to his actual age versus his apparent age, or a desire to be taken seriously by talking in such a manner. In other countries dubbings, notably Brazilian and Filipino ones, his toddler form tends to speak a lot of youngster slangs typical from such idioms.
Spirit Detective Saga
Yusuke dies, only to be resurrected by Koenma to become a spirit detective. Koenma gives Botan several items to communicate with each other, but Yusuke refuses to use them. When the Saint Beasts attack the Human World, he blindly sends Yusuke in to defeat them, without any knowledge of who they are. He also sent the tape with Hiei to give to Yusuke that described their mission to the stronghold that held Yukina. Hiei found out after giving the tape to Yusuke, so Koenma sent Kurama after them in search of Hiei.
Dark Tournament Saga
During the Dark Tournament, it is revealed that Koenma is the manager and sponsor for Team Urameshi. He also first reveals his older, adult form to look less inconspicuous to all the demons present at the tournament. For most of the arc, he is shown in his viewing box commentating on the matches, usually arguing with Jorge about something pertaining to the match at hand. In the final round against Team Toguro, he arrives at the last second before they were to be disqualified to replace Genkai after her death, as the fifth member of Team Urameshi (5 members were needed to participate in the final round.) As the matches were determined, Koenma was to go up against Team Toguro's manager, Sakyo in the fifth and final match. However, Sakyo believed that if they were to fight, it would fail to live up to the previous match; Younger Toguro vs Yusuke, and thus decided to bet his life (and outcome of his match) on Toguro's victory, provided Koenma accepted the same condition. Koenma decided to accept, believing himself no other choice as he strongly believed he would not stand a chance against Sakyo in combat, as he told Yusuke. Following Yusuke's victory and Younger Toguro's death, Sakyo confronted Koenma about their bet, convinced without his money Sakyo was no harm to him or Spirit World, Koenma told him to call off the bet. However Sakyo chose to go through with it anyway, setting off explosives all over the arena to go off within 15 minutes, before walking away. Koenma went to confront Sakyo, questioning him about his plans, which revealed he had intended to open a portal to Demon World, for no reason other than to simply make things more exciting. Sakyo assures Koenma that he has a very large ego and his plans will end with him. As Shizuru stumbles upon them, Koenma watches as pieces of the stadium collapse and kill Sakyo, before ushering Shizuru out of there. As Koenma and the others flee the exploding stadium, Jorge emerges from the rubble and joins his boss as they all safely manage to escape the danger.
Chapter Black Saga
After Shinobu Sensui begins to open a portal into Makai, Koenma explains the former spirit detective's past. He also wonders if Yusuke will be able to defeat him. Koenma was riddled with guilt, believing he should be blamed for all of Sensui's actions. Due to this, he used the power of his pacifier to revive the young Amanuma "Game Master" Tsukihito when he was killed by Kurama. He attempts to use his pacifier to seal Sensui for all eternity with the energy given out by his pacifier, called Mafuuken, but Yusuke forbade him from doing so. He was later able to activate the Mafuken when Sensui snatched the pacifier from Yusuke, but Sensui broke the barrier by using his Sacred Energy and becoming an S class. When King Enma sent down the Spirit Defense Force (SDF) to take Yusuke's body after he was killed by Sensui, Koenma went against his father and defended Yusuke's body, much to the dismay of King Enma. When Yusuke is resurrected the second time, he goes with Yusuke on Puu's back to Demon World and tells the leader of the SDF that he does not want to leave.
Three Kings Saga
By protecting Yusuke and allowing his transformation, Koenma was directly disobeying the orders of his father. Unable to return to the Spirit World, Koenma hid himself in the Human World. He warned Yusuke of Enma's machinations, and advised him to remain vigilant at all times. Upon hearing of Yusuke's inner turmoil caused by his empty defeat of Sensui and a general lack of worthy opponents in Human World, Koenma suggested that Yusuke speak with Genkai. He finds alarm in her choice in sending him to Kuroko Sanada, a former Spirit Detective, due to her lingering connection to Spirit World. Whom under his father; King Yama's orders, have slated him for erasure due to being a resurgent S-Class Mazoku.
When news reaches the Spirit World of Raizen's failing health and the shifting balance of powers, Enma reinstated his son. Koenma, back in his official robes, visits Genkai's property where Shishiwakamaru, Chu, Touya, Rinku, Jin, and Suzuka were being secretly trained at the request of Kurama. Koenma expressed that while he trusted Kurama and his intentions, the consequences of the upcoming war were too great for Kurama's personal feelings to guide his actions.
In the anime, Koenma and Botan observed the matches of the Makai Tournament. Though admitting the outcome of the tournament was a favorable one, Koenma berated Yusuke for taking too many chances with the fate of all three worlds. He promptly fired Yusuke from his position as Spirit Detective, who stated that he would have quit, anyways. Koenma and Yusuke then parted as friends. A fun note, is that Koenma's disguise at the Makai Tournament, strongly resembles, Tuxedo Kamen (Tuxedo Mask in the dub), from Sailor Moon. A character created by YuYu Hakusho's creator, Yoshihiro Togashi's wife, Naoko Takeuchi.
Botan then asked Koenma what to do with all of the "footage". Koenma told her to package it, revealing that the entire anime was a series of clips from the files of Spirit World, and the narrator was the ogre, Jorge.
In the anime, Koenma is briefly seen vigorously stamping papers with his seal.
In the manga, Koenma deposed his father and the entire ruling party of Spirit World. He investigated countless records and discovered that his father had falsified accounts of demon cruelty towards humans. While demon attacks on humans did occur, much of the violence was done at the behest of humans themselves. Humans would use demons as assassins and murderers, blaming any tragedies fully on the inherent malice of demons.
King Enma used this over-exaggeration in order to justify placing the Human World completely within the jurisdiction of the Spirit World, and blocking off the Human World from the Demon World. He would often capture low-class demons and brainwash them into committing acts of evil in Human World, in order to prove that allowing demons into the Human World would cause rampant destruction.
As a result of the deposition, Koenma became somewhat depressed. To cheer him up, Botan brought Koenma to a party, with both spirits and demons (including Enki) in attendance.
Koenma is later taken hostage along with Botan and a hundred other spirits when a radical Spirit World religious sect, in an attempt to "purify" the Human World, took over the Gates of Judgment. Unless all demons were removed from the Human World, the sect threatened to not only kill all of the hostages, but also to fire an Interdimensional Laser at the Human World. Yusuke, Kurama, Hiei, and Kuwabara managed to infiltrate the Gates of Judgement and subdued the terrorists. One terrorist explained to Yusuke that in order to stop the Laser from firing, he would need to press a specific switch on the control terminal. However, in addition to the correct switch, there were two false switches in place. One false switch would detonate the weapon and kill anyone still inside the Gates of Judgment and the other would automatically fire the Laser at the Human World. Yusuke ordered everyone to evacuate the Gates while he would decide which switch to press. However, Koenma, Hiei, and Puu remained with him while he deliberated. Luckily, Yusuke picked the right button and in the final chapter, Koenma was shown stamping papers with his seal.
- Pacifier: Koenma's powers rely solely on his pacifier, into which he has transferred his Spirit Energy, in order to conserve his power for darker times.
- It has the ability to help Koenma bring corpses back to life.
- It also immobilizes humans on contact, helping him restrain his "prey".
- Metamorphosis: He has the ability to metamorphose his body from a toddler to a young man. He frequently utilizes this technique when he appears in Ningenkai as a way to feel better about himself and to possibly prevent the ridicule/disrespect he typically gets while in his toddler form.
- Reiki Shield: Koenma has demonstrated the ability to manifest defensive shields using his spirit energy. These shields are capable of protecting multiple people at once and withstanding the corrosive aura of at least an Upper B-class apparition. This ability is seen in Episode 61: Yusuke vs. Toguro; Koenma uses it to protect Kuwabara, Kurama, and Hiei from Younger Toguro's demon energy, which had been permeating the entire Dark Tournament arena and killing the weaker demons in the audience. Koenma said "I can at least put up a barrier", having confirmed he has no combat prowess beforehand.
- Ma Fū Kan (魔封環, translated as Demon Seal Ring): Also known as the Conscience Ring and the Wicked Seal, Koenma's pacifier is the vehicle for this technique; the strongest defensive spell in all of Spirit World. When released, this technique is supposed to create a kekkai barrier net that is far stronger than the one that separated Human World from Demon World during King Yama's rule. While such an ability can be used at anytime, it is reserved for grave and catastrophic situations and darker periods of existence. This ability takes a long time, possibly centuries, before being performed another time due to Koenma needing to store an immense volume of energy within his pacifier for it to work to its full extent.
- There's a pun here, in that "Kan", when written as 看 (kan) as part of 看守 [かんしゅ] (kanshu), means "jailer". If Koenma had been able to successfully use this technique against Sensui, he would have been Sensui's eternal jailer.
- Specializing in demons, the Ma Fū Kan can even trap S-class demons, like Yomi and Mukuro, with ease. However, Sensui managed to overpower and dispel it. This was largely because Koenma used a significant amount of his pacifier's Spirit Energy to revive Amanuma, an action which Sensui had foreseen. Additionally, because the spell specializes in trapping demons specifically, it was not designed to contain Sensui's Sacred Energy. Though it could still potentially trap Sacred Energy users, as it took Sensui a great deal of effort to overcome it, it is not nearly as effective as it would be against an opponent utilizing demon energy.
- Despite his outward appearance, and childish mannerisms, Lord Koenma is actually over 600 years old.
- His birthday is October 20. His star sign is Libra, which symbolizes him being a balanced Spirit World leader.
- His usually spoken name "Koenma" is likely a pun on how he rules the Spirit World with his father, making him co-Enma.
- Koenma's punishment when he does something wrong is 100 Spankings.
- His voice actress is Mayumi Tanaka, the same who plays Dragon Ball's Krillin. They both share similarities:
- They are both short-sized (Koenma in Toddler Form) and serve as comic relief to the main character on more than one occasion.
- They are not so muscular or have that much strength but compensate this with observation and battle tactics.
- They both keep true loyal to the main character despite having all odds against them.
- They both admit to have lost to the main characters in terms of discussing when they are single-minded and just want to defeat their opponents or the "bad guys" to teach them a lesson.
- His name is Jericho in the Filipino dub version of the anime.
- In the Filipino Dub of the anime, he was voiced by the female voice actress in a similar manner to Genkai who's been voiced by the male voice actor.
- In the Latin-American Spanish Dub of the anime, Koenma had two voice actors: Azucena Martínez voiced his younger form, while Miguel Ángel Reza did the same for his teenage one.
- Koenma's Mafuuken failed seal Sensui, similar to how the Mafuba technique used against King Piccolo from Dragon Ball failed.
|e v||Team Urameshi|
|Members:||Yusuke Urameshi | Kazuma Kuwabara | Kurama | Hiei | Koenma | Genkai|