Koi Shūrai (濃い襲雷, Shurai Koi) is a shinobi from Iwagakure and is the great grandson of the Third Tsuchikage. He is also the Jinchuriki of the Four-Tailed Monkey (四尾の猿, Yonbi no Saru). Despite his status as a Jinchuriki he remains loyal to the hidden stone village and is currently serving as a Tokubetsu Jōnin, who specializes in elemental ninjutsu.
Koi is a young man of average height and build, his most distinguishing feature is his flowing red hair which is usually tied up in a ponytail. He has brown eyes and is usually seen wearing sunglasses. His standard attire consists of a black shirt, dark navy blue pants, black shoes, and a brown high collared jacket. At other times he is seen wearing the red double sleeved Iwagakure uniform underneath his brown flak jacket. He also wears the forehead protector of his home village. There are also bandages around his left hand. Besides his normal clothes, Koi also carries a large gourd on his back, which is strapped together by a wide leather band over his clothes, the gourd contains his unique blood stained ash.
He is always seen with a mischievous grin on his face, which tends to make people become wary in his presence, because of the fact that he is usually up to something. During his younger days, he wore a long black coat with the collars upturned and underneath it he wore no shirt instead he covered his chest with bandages. He also wore simple black trousers, with a white sash around the waist; he carried a small gourd which was tied to the sash, this gourd also contained his blood stained ash.
Sadistic by nature, koi is a kind of person who often tortures his enemies before landing the fatal blow. He belittles his enemies and makes fun of there weaknesses. He sees those weaker than him as mere insects he likes to toy around. Although he appears weak minded, his wits never fail when it comes in battle strategies and plans. Koi is also a prideful creature, his not someone who will just bow down to anyone, an action he thinks that only the weak will do. Swallowing his pride is the last he'll ever do, even if it means sacrificing his comrades. He wouldn't care if he hurt his allies as long as he can cut down his enemies. According to him, the word "ally" never existed on his mind but he replaced it with the word: tools. The only being in the world that Koi treats with any emotion is his wife Kasumi Kawatsuki the current Mizukage of the Hidden Mist.
As an elite shinobi from the Hidden Stone Village and Jinchuriki of the Four-Tailed Monkey, Koi is an extremely skilled shinobi, greatly known for his ability to use multiple chakra natures. He has been trained by both his granduncle Kitsuchi and his aunt Kurotsuchi both of whom were powerful shinobi in their prime and as result has gained much knowledge about the various forms of shinobi combat. Despite his easy-going and jovial nature, Koi is shown to be a very insightful and cunning man, as he seems to possess knowledge of thing that most people should not know. As one would expect of an elite shinobi, Koi possesses great intuition, able to become aware of incoming danger and react to it in time. Koi has a knack for thinking things out well in advance as well as, make decisive and accurate decisions quickly in battle, by usually taking the right course of action among several alternatives. He is also a crafty tactician, as seen from his ability to easily deceive both allies and enemies. As the jinchūriki of the Four Tailed Monkey, Koi possess a very large reservoir of chakra, and because of the four tails chakra all his powers and abilities are enhanced. Being the great grandson of the Third Tsuchikage, Koi has inherited the unique ability to fly. With the flight technique, Koi is able to fly across great distances at very high speeds.
Like other member of his family, Koi has the ability to use multiple chakra natures, and as a result he became a Tokubetsu Jōnin specializing in elemental ninjutsu. Being a native from Iwagakure, Koi's primary chakra nature is earth release, which he has shown a great affinity for, as he is able to use various high level earth release techniques. He can transform the ground into a river of mud and can also cause gigantic pillars of rock to forcefully emerge from the ground, which creates powerful shockwaves that affect the surrounding area. He is also proficient in using fire release techniques, which he mainly uses for offensive purposes. He is seen using Fire Release: Fire Dragon Flame Bullet, a technique that kneads chakra into flames, which are then manipulated into a genuine-looking dragon. Koi can also spit out a small fireball that erupts into a giant firestorm, causing widespread destruction or spit out multiple fireballs that fly wildly in every direction and assaulting the enemy. Besides his regular chakra natures, Koi also possesses two elemental Kekkei Genkai. The first is the Lava Release, which is formed by fusing earth and fire elements to create hot molten lava, the second is Ash Release, which is a product of his own creation, and is an advance chakra nature stemming from fire release.
By fusing earth and fire chakra natures, Koi is able to use the unique kekkei genkai called Lava Release. He was able to master the Lava Release, with help from his aunt Kurotsuchi, who also possessed the kekkei genkai. As stated by Koi lava release techniques are much stronger than fire release techniques and are capable of burning through or overwhelming fire release techniques. Over the years Koi has developed several lava release techniques, with Lava Release: Lava Conversion Technique, Koi is able to transform the ground around the immediate vicinity into a small lake of molten lava, this allows him to melt down anything in his path. He can also manipulate lava into various forms such as walls, domes, and globs. With his level mastery of the element, he can cause lava to erupt from the ground in the form of a geyser, which then directly attacks the opponent. One of his signature techniques when using the lava release is to transform his fist into lava and then launch it with a forward motion, much like a rocket, he calls this Lava Release: Meteor Punch.
His second kekkei genkai is called Ash Release, which is an advance chakra nature stemming from Fire Release. By burning any object into ash, Koi is able to manipulate the ash into any from and can use it to burn though an opponent’s skin or crush them underneath its pressure, though it depends on which kind of ash is being used. The user can also convert their chakra into ash and spew it out from their mouth, although it takes much more ability to create the ash than to manipulate what is already available and expel it from their mouths. He carries with him a large gourd which contains chakra infused blood stained ash, which he can manipulate in a similar manner to how a Shukaku host manipulates sand, this makes it a very versatile chakra nature. As long as there is anything to burn into ash, Koi can manipulate pre-existing ash as much as possible.
Like most other jinchūriki, Koi has the ability to create a chakra cloak by drawing on part of his beast's power, the cloak itself takes on the appearance of the Four Tails. Besides the demon cloak form, Koi can also transform in to the Four Tails full form, but is unable to control the transformed state and thus is required to let go of his control over the Four Tails, in this form he becomes more destructive and primal, due to the bestial nature of his tailed beast. When using the demon cloak all his physical attributes are increased. In addition Koi can borrow the Four Tails power to further supplement his Lava Release techniques, due to the tailed beasts own natural ability to mix its earth and fire natures, to create lava which it spits out of its mouth.
He also has increased stamina and healing abilities, which allows him to recover from major injuries usually within a day, and minor injuries within a few seconds. Mentally, Koi remains largely unaffected save for a few animalistic traits. His normal chakra level is considered high, partially because of the Four Tails. Even without directly drawing on it, a small amount of the Four Tails chakra is always mixed with his own. The Four Tails massive supply of chakra becomes an invaluable weapon for Koi, often turning the tide of a difficult battle in his favour and is useful for expanding the magnitude of his other abilities.
Koi also has a personal summon Goku, the Monkey King. Koi can summon Goku into battle in order to help him fight or distract enemies. Goku can turn himself into a diamond-hard staff, which can alter its length and width at will, Koi is quite adept at using the staff. He can depend on Goku during battle as Goku himself is quite strong and agile; he is proficient in close quarter hand to hand combat and like Koi is adept at using fire and earth release techniques. Goku is also known to be quite fast on his feet, as he can dodge attacks at close range and can also use his impressive speed to run and leap large distances in a matter of minutes. Goku is a proud warrior and is fiercely loyal to Koi.
- His appearance is based on Takaya Kagakagari from the anime/manga series Asura Cryin.
- Koi's theme song is Coming Home by Alter Bridge
- According to the author:
- Koi's hobby is creating new jutsu and picking up beautiful women.
- Koi wishes to fight against other Jinchuriki's.
- Koi's favorite foods are Fish and Chicken, while his least favorite foods are Lamb and Liver.
- Koi has completed 608 official missions in total: 183 D-rank, 270 C-rank, 40 B-rank, 100 A-rank, 15 S-rank.