Kakuzu was an S-rank Missing-nin from Takigakure and the eldest and most experienced member of Akatsuki. Most of his partners he killed due to his short temper, causing him to be partner with a newer member of Akatsuki called Hidan, who had the gift of immortality. With Akatsuki, Kakuzu acted as the orginazation's treasurer, and was largely responsible for obtaining their funds.
Kakuzu was born several decades before the start of the series, and eventually became a loyal, elite shinobi to Takigakure. He once accepted a mission to kill Hashirama Senju, the First Hokage, which met with failure due to the might of the latter's Wood Release kekkei genkai. Despite the unsurpassed danger of this mission and putting his life on line in loyalty to his village, Kakuzu's failure to kill Hashirama was met with great displeasure and he was treated with unjust cruelty. Enraged by this and feeling no further loyalty to his village, Kakuzu killed the village elders, stole their hearts, along with the kinjutsu Earth Grudge Fear, and fled with them and the village's most secret techniques. Afterwards, he became an S-rank Missing-nin and made a living as a bounty hunter. Kakuzu eventually joined the criminal organization Akatsuki, wherein he killed many of hid given partners due to his frequent rages, which eventually prompted him to be partnered with Hidan, a more recent member with the gift of immortality, who Kakuzu could never kill. Along with his other duties, Kakuzu became the Akatsuki's treasurer, and was responsible for attaining the orginazation's funds, which he did through means such as bounty hunting.
Kakuzu made his debut in the series at the sealing of the One-Tailed beast, Shukaku, alongside all the other members of Akatsuki, appearing as a silhouette. He made his first physical appearence alongside his partner, Hidan, seemingly cornering the Two-Tails jinchuriki, Yugito Nii, who had led them into a trap to kill them. Despite her using her full tailed beast form, Kakuzu and Hidan managed to defeat Yugito and take her to Zetsu, a fellow Akatsuki, before, at Kakuzu's decision, going to the Fire Teample in the Land of Fire to capture Chiriku. the head monk and a former member of the Twelve Guadian Shinobi of the Land of Fire's Daimyo, who had a large bounty on his head that Kakuzu wanted to collect. Though they killed most of the monks at the temple, one escaped to warn Konohagakure of the Akatsuki's presence. Kakuzu and Hidan travelled to a collection office hidden in a bathroom, where Kakuzu turned in Chiriku's body to Zangi, a friend of his. After he collected the money, Kakuzu went back outside and found a team of Konohagakure shinobi led Asuma Sarutobi, another former member of the Twelve Guadian Shinobi to the Daimyo, who had an even larger bounty. Kakuzu attacked Shikamaru Nara, causing the latter to release his Shadow Imitation Technique which bound Hidan, but his arrogant partner forbade him from further participation in the fight. Though he complied, he still determined to have Asuma's bounty afterwards.
Despite Kakuzu's constant warnings not to, Hidan underestimated the group and was beheaded by Asuma, surviving thanks to his immortality. Lecturing Hidan, Kakuzu overpowered Asuma and used his Earth Grudge Fear technique to reattach Hidan's head to his body, allowing him to kill Asuma with his curse ritual technique, while Kakuzu held off the others Izumo and Kotetsu, the reamining strong enemies. Reinforcements arrived not too long after and Kakuzu and Hidan were ordered to pull away by Pain, Akatsuki's leader, as it was time to seal Yugito's Two-Tau-Tails. After spending three days sealing the Two-Tails( and, in the manga, another three days sealing the Three-Tails), Kakuzu and Hidan headed for Konohagakure to retrieve Asuma's body and capture Naruto Uzumaki, the jinchuriki of Kurama, the Nine-Tails. Along the way, they were confronted by Team Asuma, led then by Kakashi Hatake, who were looking to kill them. Though Shikamaru restrained them with his Shadow Imitation Technique, Kakuzu broke free and blocked further attacks by hardening his body. He was then, however, impaled through the heart by Kakashi. Though this would normally be fatal, Kakuzu survived due to his supply of five hearts, and repelled Kakashi into a tree, causing it to collapse several others as the latter attempted to attack Hidan.
With the enemy stunned at his survival, Kakuzu unleashed three monsters from his body that each contained one of his hearts and each a certain chakra nature to attack separately on Kakuzu's will. Fighting together with hidan after this, the Akatsuki duo proved to be more powerful than their opponents could handle as a pair, as Kakuzu nearly killed Kakashi and the others multiple times. Realizing this, Shikamaru decided to lead away Hidan and deal with him separately. After he left, Kakuzu continued to battle Kakashi, who was backed by Shikamaru's teammates, Ino and Choji. He overpowered Kakashi and attempted to steal his heart to replace the one Kakashi took from him, but fortunately Shikamaru managed to trick Hidan into using Kakuzu's blood in his ritual, which caused his main body's heart to be destroyed. The monster with the Lightning heart then allowed Kakuzu's body to absorb its heart, reviving him and he had the Fire and Wind element hearts rejoin his body, causing him to become more monster-like and powerful. He then caught and nearly killed Kakashi and the others, but thankfully Team Seven arrived and Naruto and Yamato managed to counter Kakuzu's attack. Naruto proceeded to prepare his new Wind Release: Rasenshuriken technique, which Kakuzu noted would kill him if it hit him. Though the attempted attack failed, Kakuzu didn't succeed in capturing Naruto's heart(completely ignoring that Naruto had to be take alive). As Naruto prepared to try again, Kakuzu switched over to a long-range attack style, and seeming killed Naruto, but the latter had switched with a clone, and successfully struck Kakuzu from behind with the Rasenshuriken. Left with his last heart on the verge of failure afterwards, Kakuzu lamented being defeated by what he saw as children, but Kakashi assured him that being surpassed by the future was natural, before finishing Kakuzu off with his Lightning Cutter. Later, he was given an autopsy by Tsunade, who declared Rasenshuriken a kinjutsu.
Kakuzu was later revived by Kabuto Yakushi, alongside his fellow deceased Akatsuki members, as a military weapon for Aktsuki's side in the Fourth Shinobi World War. Kakuzu ended up at the coast of the Land of Lightning, where he emerged from within the ocean along with the White Zetsu Army. Replinishing his supply of hearts, Kakuzu witnessed Kinkaku's jinchuriki transformation and remarked he hadn't seen it in a while. After he found himself confronted once again by Team Asuma, Kakuzu took Hidan's absence as proof of his survival against Shikamaru, before regaining his fifth and final heart. Dsipersing the heart monsters from his body, Kakuzu had his arms sliced off by Izumo and Kotetsu, who he stated he tended to forget shinobi who didn't have a bounty on their head to. Escaping capture while being pursued, Kakuzu's Lightning heart monster was destroyed by Tenten using the Bashosen of the Gold and Silver Bothers. Battling Darui, Izumo, and Kotetsu, Kakuzu was shocked at the sight of Choji's chakra butterfly wings overshadowing the battlefield and was restrained under the latter's giant fist. Later bound by the cloth used for sealing, Kakuzu was nearly freed by the Outer Path statue, but was sent back into the afterlife, along with all the other revive shinobi besides Madara Uchiha, following the release of the Impure World Reincarnation technique.
Kakuzu was a very greedy man, who loved money second to nothing and often measure tasks by the highest profit he could attain from them. He openly stated that he tended to forget shinobi who were worth nothing, and thought of money as the most important thing. However, Kakuzu was very loyal to Akatsuki, and was more than willing to follow orders, as when Pain, his leader, ordred him and Hidan to fall back to seal Matatabi, the Two-Tails, he complied at once, regardless of the high bounty he could attain from Asuma Sarutobi. Kakuzu was very short-tempered and killed many of his partners in Akatsuki as a result, which eventually led him to be paired with Hidan, who he could never kill. Kakuzu and Hidan despised one another a lot, but got along well when they were both in the killing mood, and their teamwork capability was near flawless. Kakuzu respected shinobi who were powerful and who would think out strategies, as he openly praised Shikamaru Nara's intellect and repeatedly warn Hidan not to underestimate powerful oppontnents, often teasing him when he did.
As a member of Akatsuki and an S-rank Missing-nin, Kakuzu was an exceptionally powerful shinobi, who was able to single-handedly kill many of his partners in Akatsuki when he became angry and survive a battle with Hashirama Senju, the First Hokage, considered the strongest shinobi of his time, before attaining his Earth Grudge Fear technique, which later became his main source of power. Kakuzu was easily more powerful than his last partner, Hidan, and when the two worked together, due to Hidan's immortality and Kakuzu's lack of worry about killing him, the two were practically unstopable and their teamwork was almost flawless. Working together, the two managed to defeat Yugito Nii, the Two-Tails, Matatabi's jinchuriki and an immensely powerful kunoichi, despite her transformation into her tailed beast. Due to being the eldest and most experienced member of Akatsuki, Kakuzu was capable of fighting against multiple opponents at once while controlling the pace of the fight, such as when he battled Kakashi Hatake, Choji Akimichi, and Ino Yamanaka at once, and nearly killed them all single-handedly, despite Kakashi's considerable powers and his partners great teamwork abilities, only failing thanks to Naruto Uzumaki and Yamato's timely intervention at the vital moment.
Kakuzu's main source of power was his Earth Grudge Fear technique, which was a kinjutsu that he stole from his village at the time he defected. Employing it allowed a series of threads to embed in Kakuzu's body that allowed him to both repair his body when hurt and stitch up others when they were hurt, with his partner Hidan being his most frequent patient, but also allowed him to steal hearts and other internal organs from people and shinobi, allowing him to prolong his life and gain more chakra natures, as well as immensely increasing his chakra supply. Kakuzu could cause monsters to emerge from his body with his extra hearts inside them, each one capable of attacking on his will with a specific chakra nature. His Earth Grudge Fear technique allowed him to incorperate all the chakra natures(in the anime, though he only got four in the manga, he also got water, which he could use to create a water wall to block attacks and use the Water Clone technique while battling Kakashi). With Earth Release, Kakuzu could harden his body and withstand powerul attacks, with only Kakashi's Lightning Cutter being strong enough to pierce him. Using this together with taijutsu, Kakuzu was able to punch down the massive seal gate at the Fire Temple in the Land of Fire. Using Lightning Release, Kakuzu could use Lightning Release: False Darkness to blast an enormous wave of lightning that took Kakashi two or three Lightning Cutter uses in a row to catch and when the monster with this heart emerged from within Kakuzu, it could shoot bolts of lightning from its mouth at his will at any target. With Wind Release, Kakuzu could use Wind Release: Pressure Damage, which easily overpowered Kakashi and nearly killed him with one use. When the monster with this heart emerged from within him, it could fire barrages of Wind Release balls that could easily pierce a thick tree. With Fire Release, Kakuzu could use Fire Release: Intelligent Hard Work to create the largest fire technique seen in the series that incinerated many trees in an instant. When this heart's monster emerged from with Kakuzu, it could fire off streams of fire. The Fire and Wind Release monsters could form together and create a uniform attack that mixed Fire and Wind Release to an almost unstoppable degree. Using all this together, he was able to overwhelm Kakashi Hatake and most of Team Asuma.
Kakuzu was a powerful taijutsu fighter, able to catch an enlarged punch from Choji Akimichi, who had enlarged his arm with his expansion technique, and repel Kakashi Hatake into a tree and cause several others to collapse with a single kick. He had incredible raw speed, able to easily move short distances in an instant and disappear from unfocused eyes. His taijutsu skill was easily enough to equal Kakashi, who himself is very skill with taijutsu. Kakuzu had a very keen annilytical ability, able to easily figure out Shikamaru Nara's shadow techniques and their purpose, and devise a way to counter them. This particular skill was partly because of his unrivaled experience.