Kakuzu is 91 years old and a former ninja from Takigakure. His village sent him on a mission to kill the Leaf village's first Hokage, Hashirama. Kakuzu failed and returned to his village gravely injured, but instead of honoring him they cruelly punished him for his failure and imprisoned him. Filled with rage, he escaped once he recovered and stole the scroll containing his village's forbidden jutsu where he learned "Earth Grudge Fear", which he used to take the hearts of the village leaders before leaving the village.
Service in Akatsuki
Sometime after leaving his village, Kakuzu met up with and joined the criminal organization called the Akatsuki. Throughout his years of service, though, Kakuzu managed to kill his partners due to the large level and high potency of his jutsu, as well as the fact that he's basically immortal. He was eventually teamed up with Hidan who was another immortal, and soon they became known as the "Immortal Duo". In current times, they were tasked to capture the Two-Tailed Beast housed within Yu'gito Nii. As she led the two into a cavern and trapped them there, she underwent Tailed Beast transformation, but because she wasn't aware of Kakuzu and Hidan's abilities, she ended up on the losing end of the fight and was captured.
In search for another jinchuuriki, Kakuzu and his partner traveled to the Land of Fire and the Fire Temple within. After Kakuzu busted down the gates, they were stopped by the head monk, Chiriku, and a battle ensued. It turns out there was a very large bounty on Chiriku and that's why Kakuzu actually wanted to fight him. After defeating all of the monks, they left with Chiriku's to a collection house to collect his bounty.
The two would later encounter four Leaf ninjas: Asuma, Shikamaru, Kotetsu, and Izumo who were trying to capture the pair. Hidan wanted to fight them alone and insisted that Kakuzu didn't interfere. Hidan caught Asuma in his curse technique and slowly inflicted injuries to him, but was stopped by Shikamaru before he could deal the final blow. Shikamaru forced Hidan out of the technique and Asuma cut off Hidan's head, which Kakuzu re-attached to the shock off the Leaf team. Kotetsu and Izumo decided to attack Kakuzu, Izumo using "Starch Syrup Capture Field" to keep him in one place while Kotetsu summoned a giant conch-looking mace, but both were caught in Kakuzu's grip thanks to his extending limbs.
Suddenly Aoba Yamashiro came to help and used "Murder of Crows Confusion" to blind Kakuzu while his partner Raido Namiashi tried to cut off his head, hiding in the flock using a black cloak and black sword. However Kakuzu used his "Earth Spear" to protect himself and retreated back. During this, Hidan once again used his curse technique to fatally wound Asuma. Although Hidan wanted to finish the fight and Kakuzu wanted Asuma's bounty, the duo left when Akatsuki leader Pein ordered them to come back to their hideout.
Kakuzu and Hidan would later be caught in a trap orchestrated by Shikamaru and the team of Kakashi, Choji, and Ino. Shikamaru had Asuma's property absorbing trench knives which he used to absorb his shadow abilities. He threw various kunai at the duo but Kakuzu used his Earth Spear to defend against them, however he and Hidan were frozen in place when Shikamaru stabbed their shadows with the trench knives. Although unable to move, Kakuzu was able to remove the knife from his shadow with his tendrils, just in time to avoid Hidan's attack as he was being manipulated by Shikamaru's technique. Shikamaru had Hidan continue to attack Kakuzu while Choji attacked them both, prompting Kakuzu to once again use his Earth Spear technique. Kakashi attacked Kakuzu with his "Raikiri", piercing his hand through Kakuzu's hardened chest, at the same time he collected some blood from him and put it in a vile.
After Kakuzu went down, Kakashi aimed for Hidan but was quickly kicked aside by Kakuzu, revealing that what Kakashi hit was a spare heart on his back. Kakuzu then released the rest of his hearts who had black tendrils for bodies. Shikamaru's technique then wore off and Hidan attempted to hold Kakashi down while Kakuzu's wind heart used "Pressure Damage" on them, with Hidan's immortality preventing him from death. However, Kakashi would escape the blast and would continue to battle the combination of Hidan and Kakuzu's Wind and Fire hearts.
Meanwhile Kakuzu attacked Ino and Chouji while Shikamaru ran from Kakuzu's lightning heart, Kakuzu stopped Choji's punch and slammed him into a tree, then proceeded to grab him and Ino and try to choke them. Shikamaru however saved them by unsealing a scroll full of water that caused Kakuzu to be electrocuted by his own lightning heart. Shikamaru then threw kunai with exploding seals but Kakuzu blocked them with "Water Encampment Wall". Kakuzu was about to hit them with "False Darkness", but Kakashi used a "Shadow Clone" to trick Hidan and arrived to save the team from the electrical heart's attack by deflecting it with his "Raikiri", then secretly gave the vile of Kakuzu's blood to Shikamaru who used his Shadow Imitation technique to take Hidan deep into the woods.
Kakuzu and Kakashi engaged in close combat, Kakashi managed to get Kakuzu with a pile driver, but it turned out to be a "Water Clone" of Kakuzu and he came out of hiding and attacked. Kakuzu quickly gained the upper hand over Kakashi, meanwhile Hidan got out of Shikamaru's "Shadow Imitation" and regained control of his body, allowing him to draw some of Shikamaru's blood and activate his curse technique and impaled himself in the chest... at the same time, Kakuzu's heart was destroyed just as he was about to take Kakashi's heart to replace the one destroyed earlier. Shikamaru revealed that Hidan didn't draw blood from him, but rather he doused Hidan's weapon with Kakuzu's blood instead. The masked hearts rushed to aid Kakuzu and, despite Choji's attempt to stop them, the three re-integrated into Kakuzu, replacing the destroyed heart with the Lightning one and attacked with a combination of his Intense Pain and Pressure Damage jutsus, but Kakashi, Ino, and Choji were saved by the arrival of Yamato, Naruto, Sai, and Sakura. Naruto and Yamato used a combination of a wind and water jutsu to block Kakuzu's attack. Sai and Sakura were sent to aid Shikamaru while Yamato and Naruto remained to aid Kakashi and the others.
Naruto used his "Shadow Clones" to swarm Kakuzu, but he destroyed them but was almost struck from behind by Naruto's new technique, the "Rasenshuriken", during the confusion. Kakuzu leaped away and Naruto failed to connect. Kakuzu then went in for the kill, but Naruto was saved by the combined efforts of Kakashi and Yamato, forcing Kakuzu to evade Yamato's attack while Kakashi cut Naruto free from Kakuzu's tendrils.
Naruto used the same trick again of swarming Kakuzu with "Shadow Clones" to distract him so that he could try the "Rasenshuriken" again. Kakuzu realized that Naruto is a close range brawler and that the "Rasenshurken" was a dangerous technique, choosing to ignore the Shadow Clones, going straight for the kill before the "Rasenshuriken" could hit him... but he didn't anticipate the one with the "Rasenshuriken" that he just "killed" was actually a Shadow Clone and the real Naruto struck him from behind with the "Rasenshuriken", destroying the two spare hearts on his back.
Kakuzu wasn't dead though, just paralyzed. After expressing his shock that he was defeated by kids, Kakashi replied that the youth eventually surpass the old and then dealt the final blow while Kakuzu laid immobilized. His body was then brought back to Konoha where Tsunade conducted an autopsy to discern the Rasenshuriken's affects.
Fourth Shinobi World War
Kakuzu's eternal slumber would be interrupted though, as Kabuto, after absorbing Orochimaru's powers, was able to bring him and others back using "Resurrection to the Impure World" in a bid to win Madara Uchiha's favor. Upon succeeding, from orders by Madara, Kabuto sent Kakuzu and the other resurrected to war with the Allied Shinobi Forces. On the battlefield, he killed many ninja and took their hearts, housing them in his masks. He then briefly encountered Ino, Shikamaru, and Choji again after they aided Darui and others in capturing Kinkaku and Ginkaku. Kakuzu mentioned how the alliance had captured some of their warriors, but made them aware that Asuma is now on his side, whom they engaged in battle. Kakuzu sent his masks off to fight on the battlefield while he engaged Darui, Kotetsu, Izumo, and others in battle. He was ultimately pinned down by a giant Choji.
Kakuzu is made up of the younger body parts of others that he has defeated and is held together by the seemingly endless black tendrils within his body, created from the "Earth Grudge Fear" jutsu. These tendrils are able to pierce flesh and are strong enough to break bone. He is able to extend parts of his body by using these tendrils and can also extend the tendrils by themselves to snare his enemies, he has even used these tendrils to re-attach the severed body parts of Akatsuki members.
In the Narutoverse people's chakra possess a single elemental affinity which allows the individual to learn that specific element easier than any other. Using different elemental jutsus is done by recomposing their chakra into the element they want, which is not easy to learn and not as powerful as their affinity. Attached to Kakuzu's back by his tendrils are four masks, each shielding one of four spare hearts that he has taken from enemy ninjas. He is not only able to use these spare hearts' elemental affinities, but he is able to use them to replace his heart if anything happens to it. They can also act independently of Kakuzu and he can give these masks pseudo-bodies made out of his tendrils.
He has displayed a few elemental Jutsus. "False Darkness" takes the form of piercing lightning javelins that can range in numbers; "Searing Migraine" carpet bombs the terrain in fire; "Pressure Damage" a wind vortex of highly condensed air; "Earth Spear" where his body is transformed into nearly indestructible adamant. He could also combine his wind and fire elements to create a powerful inferno. He also used two water jutsus; "Water Encampment Wall" to create a shield made out of water, and "Water Clone" where he created a duplicate of himself out of water.