Joker is the 3rd Strongest of the Jyushinshuu, and only weaker than Raiha when he uses the Raijin.
Joker has a strange sense of fashion. In his first appearance, he showed himself in the form of a caped jester. In the next arc, he appears wearing a baseball cap, and normal clothing. And last, he was seen wearing a beanie, as well as a sleeveless shirt, his dark hair now blond. Whatever clothing Joker wears, his eyes are always covered.
Joker's personality for the most part is a mystery. When appearing in the series for the first time, he seemed to be a kindly, although silly person who offered advice to Hokage at times. In a flashback, Joker stated that he valued his freedom above all else and that he didn't grow attached to anything. He joined Uruha on one condition, that he could address Kurei as an equal, to which the latter agreed. However, Joker seemed to change somewhere along the way as he had an outburst when explaining to Hokage that he didn't believe there was anyway Kurei could have been killed. In many ways, Joker is like an adult, but not so much more mature version of Kaoru.
Part in story
He initially gets involved with the Uruha through attempting to fight Jisho, one of the members of the Jyuushinshu, and saying that he intended to kill all the members of the Uruha. He fights Jisho continuously for two days, and then meets Kurei only to reveal that he wanted to join the Uruha in the first place. Kurei then offers him a spot in the Jyuushinshu, and Joker joins on two conditions: that he can live the way he wants and can leave whenever he wants to, and for him to be allowed to be informal since it "pains him to be respectful." Years later in the Ura Butō Satsujin, during Hokage's battle against Uruha Oto, one of the Jyushinshuu, Neon, attempted to destroy the dome with her ultimate attack Requiem. It is here Joker makes his entrance by saving Sakura Menou from the falling ceilings. He 'befriends' the Hokage and keeps dropping hints on the opponent's power. Some, however, is a bit aware that he seems to have a double agenda.
This comes true when he accidentally butts in when Kaoru Koganei and Saichou are ambushed by J-Keeper. While he first fought for the two, when Koganei is struck by an attack, Joker suggests a double attack along with Saichou. However, instead of cooperating, Joker fatally wounds Saichou and obliterated J-Keeper by himself. When Recca Hanabishi finds out about this, Joker reveals that he is a member of Kurei's Uruha Kurenai, not J-Keeper as he is a mere pawn all along.
During the finals, he still jokes around. Eventually his turn comes when he's about to face Koganei. He reveals the power of his Taishaku Kaiten, the madogu to control gravity. However, Koganei eventually unlocks the final form of his Kougon Anki, which greatly injures Joker. Instead of continuing, he conceded the match and watches as Recca and Kurei's final battle ensues, ending with the victory in Recca's side.
Joker reappears in a much friendlier note later (though still being put under suspicion in the beginning) and helps the Hokage team find the cavern which leads to where the madogu, Tendou Jigoku (which is said to grant its wielder eternal life), was hidden. He then disappears again to rejoin Kurei.
In the last arc of the manga, Joker reappears with Kurei, Raiha and Neon. He first appears to stop Kurei from burning Saichou after he burnt a member of the Ura Uruha Shishiten, Kadotsu. He later waits for Koganei because he wants to fight him, but Koganei avoids him. Eventually they both fight against one of the Ura Uruha's Shishiten, Kirito and her pet Shiguma, and they manage to defeat her. As they are about to depart, Kadotsu reappears and manages to stab Joker. Joker then proceeds to open up a black hole, which pulls Kadotsu and himself into the void. As he is sucked in, he leaves his Taishaku Kaiten behind. Kurei sensed that Joker sacrificed himself against Kadotsu, as Kurei murmurs his name as Joker was getting sucked into the black hole.
Abilities and Techniques
Taishaku Kaiten (帝釈廻天, Emperor's Domain) is a deformed looking trident-ax madōgu with a single eye below it's blades Joker can manipulate gravity within a fixed radius. Taishaku Kaiten can also be used to increase or decrease the weight of objects by manipulating its mass. He can also move swiftly by reducing the effects of gravity on himself. The power to create a blackhole is said to be his weapon's ultimate power.
It has been speculated that Nanashi of Marchen Awakens Romance (MÄR) (another series created by Nobuyuki Anzai), is actually Joker, who ended up in the world of Marchen Awakens Romance after he got sucked into the black hole in the latter part of the Flame of Recca manga. His hair closely resembles Joker's hair in the last part of the manga and Nanashi sports the same bracelet Joker wore when he got sucked into the black hole. While a common feature in many manga, they both have one fang as well.
Additionally, it is also hinted that Nanashi is not from the world MÄR-Heaven. According to Babbo, a character in MÄR who has a strong sense of smell, Nanashi has the same scent as Ginta Toramizu, who's from another world. Like Ginta, Nanashi also has incredible strength, due mainly to MÄR-Heaven's lower gravity than other world's.
In chapter 36, when Nanashi uses his electric power, he says that it makes him feel like he has forgotten someone, showing a silhouette strongly resembling Raiha. In chapter 106, another MAR character, Galian, appears, who has a resemblance to Raiha. At the same time, it was revealed that Nanashi was originally found by Galian with a stab wound in his stomach, which matches the one Joker recieved from Kadotsu.