They are described as an elite group of fighters, with each member possessing energy levels equal to mid/upper A classes, however one member has stated that even the strongest A class are no match for them. However, even combined, they have no chance against even a single, low level S class being. They are feared throughout the Demon World as "Hunters".
The S.D.F. has two primary functions, as stated in the manga. One is to patrol and secure the boundaries that are controlled by Spirit World, and to lead operations to further Spirit world's influence in the Demon world. Their secondary task is to monitor any demons residing in the Human world who possess A-class or more power, and to eliminate them if they pose a threat to the safety of the Human and/or Spirit world.
It is hinted that the group has been around for a long time, as one of the members bragged about being responsible for injuring the legendary thief Yoko Kurama , though a direct time frame has never been given. Their first appearance was during the Chapter Black Saga, where they were sent to Hangman's cave (Demon's Door cave in the English dub). The group acts in a militaristic type fashion, with one member being appointed by Emperor Enma as the leader of the group, who the rest refer to as "taichou" or captain in the English dub. Because of their militaristic demeanor, the group carries out their orders with unerring precision, even willing to go so far as to physically subdue Koenma and assault Kiyoshi Mitarai (knocking him unconcious) in order to carry out their assignment to destroy the body of Yusuke Urameshi. It is their leader, who reports directly to Emperor Enma, that informs Koenma of Yusuke's demon heritage when asked why they were sent to kill Yusuke. This plan fails, however, when a transformed Puu arrives and covers Yusuke's body, which is already over-flowing with youki (demon energy).
A resurrected Yusuke manages to incapacitate the leader with just one blow (a kick to the face), then uses his new demon powers to intimidate the other members into letting Koenma go. Once Koenma and Yusuke leave to track down Sensui and the others in Demon world, the group members gather together to ask their captain what is going on. Confused, as they were originally under the assumption that they were the strongest of all of Spirit world's army, and that their combined powers could take down even the strongest A-class demons. However, in just one visit to the Human world, they've come across two beings, one human (Sensui) and one demon/human hybrid (Yusuke), who each possess greater power than all of them. Their captain responds by informing them that while they are indeed the strongest fighters Spirit world has to offer, there are beings in the Demon world that possess the potential to hold even greater power than them. This causes some of the members to panic, as their confidence in their abilities is momentarily doubted, until the captain orders them to calm down. He then reminds them that the Demon world is a place that represents overwhelming offense, while the Spirit World represents impenetrable defense.
The captain then orders them to band together to seal the tunnel created by Itsuki that leads to the Demon world, giving them 48 hours to complete a task that would normally take a week to accomplish. Despite the short time frame, the group is able to accomplish this task, though not before Koenma and the others return to the Human world. The group confront a still transformed Yusuke, along with Koenma, Kurama, Kuwabara, and an unconcious Hiei outside of the cave's entrance, where the leader warns Koenma that his father will be informed of his interference with their other mission. Koenma boldly states that that's fine with him, and tells them to also inform his father of his decision to remain in the Human world for now, seemingly placing himself in temporary exile, until the assassination order on Yusuke is lifted. The S.D.F. are next seen outside of Genkai's temple, where they are preparing to open a path for Yusuke to travel to the Demon world.
Yusuke remarks how only three members of the group are able to accomplish in moments, what it took someone as powerful as Sensui weeks to perform, even though Ituski, Sensui's demon partner, was directly responsible for the creation of the original tunnel, and not Sensui. The group is this time led by a new captain, as the former captain was replaced when their original mission to kill Yusuke failed. Prior to Yusuke leaving, Kuwabara, who is deeply angered and feeling betrayed by Yusuke, Hiei, and Kurama for wanting to go the Demon world and fight in an upcoming war (which would potentially pit them against each other, an idea none of the three are bothered by), accuses the S.D.F. and by extension, Emperor Enma, of using the conflict in Demon world to get rid of Yusuke, Hiei, and Kurama, whom they could not kill on their own. The new captain doesn't deny this accusation, and tells Yusuke just before he leaves not to come back. Afterward, they open separate paths for Hiei and Kurama, who each leave after Yusuke, before returning back to the Spirit world. They are then absent throughout the remainder of the series.
As they act as a military group for the Spirit World, each of them wear identical uniforms, consisting of a multi-colored vest with shoulder pads, which is worn over a long-sleeved white shirt, elbow pads, and loose-fitting gray pants which wrap around the heel of their black shoes. They also wear gloves, which, like their elbow and shoulder pads and shoes, are colored black.
The vests are a deep gray, with yellow lines running from each side of the collar to across the shoulders, as well extending from the front of the collar straight down the mid-section, and connecting to a yellow line extending around the bottom of the vests. There is also a yellow line that extends from the left shoulder down diagonally, meeting at the point where the line extending down the mid-section meets with the line extending around the bottom. A red circle is also imprinted on the left side over the chest area, though it is unknown if this is just part of the vest's design or if it performs some special function. In the series (both anime and manga), seven of the nine members are mentioned by name, those being Ōtake, Saitō, Sōrai, Ōhō, Ryūhi, Rinbai, and Shun-jun. The remaining two are left anonymous, and their names are not mentioned.
Each member of the SDF has the ability to open or close dimensional portals and holes, flight, as well as fire blasts of Spirit Energy from their hands. None of the members have demonstrated any powers unique to them. While each of them are easily Middle to Upper A Class, they are helpless against even the weakest of S Class Demons
- Ōtake (大竹) is as a middle-aged brown haired male, with a bang hanging down across the left side of his forehead, just above his left eye. He is the former captain of the S.D.F., possessing a commanding presence, as seen when he stops the other members from panicking after Yusuke's revival, as well as a stern sense of loyalty. Though he is respectful to Koenma, even in times when his authority overrides the prince's, he is nonetheless a man focused on getting the job done as quickly and efficiently as possible. He is later removed from his position as captain by Emperor Enma, and is replaced by his subordinate, Shun-jun. Towards the end of the series, he is seen being involved with a renegade faction in Spirit World who overtake the majority of the Spirit World palaces occupants hostage (including Botan), and attempt to use a spiritual weapon to cause damage to the Human world. Yusuke, Kuwabara, Hiei, and Kurama gather once again are able to take down and subdue the group, as well as prevent the weapon from being fired. While he becomes a traitor in the manga, in the anime, he is portrayed much more sympathetically, and even defends Koenma's decisions in helping Yusuke, despite his opposition to them. He is voiced by Hidetoshi Nakamura in Japanese and Troy Williams in English.
- Saitō (才頭) is a male in his mid-to-late twenties, and has long dark hair extending down a little past his shoulders. As a member of the S.D.F. he shares similar traits to the others, expressing obedience by following the orders of his captain without question. This is seen when Ōtake asks him how long it will take for he, Sōrai, and Ōhō, to seal up the tunnel, to which he replies that it will take ten days with their combined efforts. Ōtake gives him a week to accomplish it, and Saitou readily agrees. He is voiced by Mitsumasa Kishimoto in Japanese and Merk Harbor in English.
- Sōrai (草雷) is a bald male in his mid-twenties, and is easily the shortest member of the S.D.F. Although he doesn't have much dialogue in the series, he is seen as one of the first members to panic upon Yusuke's resurrection, wanting to know how there could be beings who possess greater power than the S.D.F. He has also been shown as being obedient, like the others, as he follows the orders of his captain with little question. His voice is provided by Tsutomu Takayama in Japanese. In the English dub he was voiced by Ricky Page in the original release and replaced by Joel McDonald in the Blu-ray remaster.
- Ōhō (翁法) is one of only two females in the S.D.F., and is distinguished by her shoulder-length brown hair, with three bangs that hang down over one side of her face, just above her eye. She also is tall, thin, and fair-skinned, giving her an appearance that is oddly somewhat similar to Shizuru Kuwabara. Like the other members, she is prone to following the orders of her commander, however, she is also one of the few who spoke out after Yusuke's resurrection. She is voiced by Ayumi Kida in the Japanese version. In the English dub, she was originally voiced by Melissa Ellis for the original release and later by Alexis Tipton for the Blu-ray rerelease.
- Ryūhi (俠妃) is the second of the two females in the S.D.F., is characterized by her pale-blond hair, which is kept up by a violet ribbon she has wrapped around her forehead. She is of average height, and possesses an athletic build. Like the others, she is loyal and follows orders without exception, though she was outspoken when Yusuke's resurrection revealed him to have S-class level power. Her voice is provided by Kyoko Hikami in the original Japanese. In English she was voiced in the dub's original version by Clarine Harp and later by Stephanie Young for the Blu-ray remaster.
- Rinbai (倫霞) is a male in his late twenties, and is slightly tall in stature. He is characterized by his dark-violet hair, which is worn in a fashion similar to a mohawk, and extends down past his shoulders. He also sports two red lines on both sides of his face, and wears a stern expression. Though he doesn't speak, he is quite loyal to his commander, following orders without question, and is also one of the few who seemingly doesn't panic after Yusuke's resurrection, though he is initially disturbed by it during their brief encounter in the tunnel. He is voiced by Koji Ishii in Japanese and by Bob Carter in English.
- Shunjun (舜潤) is a male in his mid- to late-twenties, and is characterized by his light blue hair. Initially, he is depicted as a reserved individual, as he doesn't speak and maintains a cool demeanor during their first appearance in Hangman's Cave. However, he is later shown to be a bit of a braggart, as he remarks to his two comrades about how the S.D.F. is feared among demons, and considered their "boogey-men". It is also insinuated by one of his teammates, Rinbai, that he was responsible for the injury that forced Yoko Kurama to possess the unborn body of Shuichi Minamino, who later became one of Yusuke's allies and teammates. Shun-jun is later seen as the replacement commander for the former captain Ōtake, who was removed from his position for failing to kill Yusuke. Shunjun is voiced by Kazuaki Ito in Japanese and by Troy Baker in English.
- Two unnamed members: Though their names are never mentioned, they wear the same standard uniforms as the other members. One is characterized by one being a tan-skinned male in his mid- to late-twenties, possessing a dark, bowl-cut hair style and a stout nose. The other is characterized as being very-tall, possibly the tallest out of the group, and possessing copper-colored hair, and is seen as also having stout facial features as well. These two remain in the Hangman's Cave with Captain Ōtake, and are initially the ones assigned to carry out Yusuke's assassination, though Saitō, Sōrai, and Ōhō attempt to assist as well when Yusuke's body begins emitting Demon Energy. These two seem to be the harshest and most sadistic members as they beat up Mitarai while mocking him, despite his change of heart. They also seem to be the most cowardly, as one of them quickly begs for mercy in the English dub. In the Japanese version the shorter one is voiced by Atsushi Ishida in Japanese and Jeff Johnson in the English dub. The taller one's Japanese actor is unidentified but in English he was originally voiced by Jim Foronda and later redubbed by Scott Freeman for the Blu-ray rerelease.
- Due to the general appearance, the group was inspired by the “Science Special Investigation Squad” (aka the Science Patrol) from the Ultraman series. There is also obvious Super Sentai inspiration within the group. Yanagisawa at one point even mentions in the manga that he saw a bunch of guys looking like the Science Patrol entering the cave.
- Ōtake resembles Johness the Dissector and Light Nostrade, two characters in Hunter x Hunter.
- The Spirit Defense Force were Not Villains in the Series, they were presented as minor characters in the Chapter Black Saga. Then, in the fourth and final season of YYH/Saga of the Three Kings, Saitou, Sourai, Oohou, Ryuuhi, Rinbai and Shun-jun (except for Captain Otake) were now Urameshi’s allies, but they weren’t seen throughout the whole entire series.
- 1) Yu Yu Hakusho manga, volume 16, chapter 09.
- 2) Yu Yu Hakusho manga, volume 16, chapter 10.
- 3) Yu Yu Hakusho manga, volume 17, chapter 05.
- 4) Yu Yu Hakusho manga, chapter 158.