Katsuyu

is a resident of the Shikkotsu Forest and the personal summon of Tsunade and Sakura Haruno.

Background
At some point during the Second Shinobi World War, Katsuyu was summoned to the battlefield, alongside Gamabunta and Manda.

Personality
Compared to Gamabunta and Manda, the summons of Jiraiya and Orochimaru respectively, Katsuyu is quiet and more subservient to its master. It addresses Tsunade with the highly respectful honourific "-sama", and, in the English versions, calls her Mistress or Milady. Katsuyu tends to also use honourifics like "-kun" and "-san" on other villagers. Sakura and Shizune show it considerable respect, addressing it as "Katsuyu-sama". In the English dub, it is referred to as "Lady Katsuyu". Katsuyu is also shown to have a playful side and a jovial friendship with Tsunade as seen in the omake at the end of Naruto: Shippūden episode 118. It tells Tsunade of the way Sakura overreacted when it fabricated a story of its difficult journey (including: the physical limitations of its slug body, falling off a rope bridge, and being attacked by villagers) in reaching Team Three during the Three Tails Arc. Tsunade comments on Sakura's naivete and they both share a chuckle at Sakura's expense.

Appearance
Katsuyu is an enormous entity whose true size is unknown. In fact, unlike other summons which may be multiple in number as well as vary in size, the slugs summoned by Sakura and Tsunade are all in fact portions of the same entity. The general size of the summoned portion of Katsuyu is easily as tall as the largest building in Konoha. It is predominantly white in colour with three blue streaks that run vertically down the middle and side of its body from its head and tapering off at its tail. Its optical tentacles have a slight tint of grey to them and it also has two sensory tentacles on either side of its mouth.

Abilities
Offensively, Katsuyu can spit acid using its Tongue Tooth Sticky Acid technique, which is strong enough to melt through rock with ease. With its Slug Great Division, Katsuyu can divide itself into smaller clones to evade attacks or to serve small scale functions, and is capable of quickly reforming itself if need be. Katsuyu's clones can attach themselves to injured people, allowing its summoner to channel their chakra through it to heal the injured person. Katsuyu's clones can also telepathically share information with each other which allows the slug to serve other supplementary purposes when divided such as gathering and dispensing information. Katsuyu could also put Tsunade's bisected form back together, although due to Tsunade being weakened, the process took a considerable amount of time as Katsuyu needed Suigetsu to bring the two halves of her body together before Katsuyu could rejoin her.

Furthermore, due to its genetic and physical make up, Katsuyu is extremely resistant to damage, as seen during Pain's Assault. Katsuyu is also able to absorb people into its body, thus protecting them from physical impacts like the large-scale Shinra Tensei using its own high durability. A palm-sized version of Katsuyu even survived the corrosive chakra of Naruto's eight-tailed form and Nagato's Chibaku Tensei. Oddly enough, though slugs are typically slow, Katsuyu's clones were fast enough to throw themselves in front of Shizune, Ino and Inoichi before the Animal Path's rhino summon even managed to hit them. Due to its ability to divide, a summoner is able to separately summon her divisions so portions of Katsuyu can be in more than one place at any given time. Also due to this physical make up, the slug is able to alter its bodily form and is able to deconstruct entirely into nothing more than a pool of body fluids. This does not render it incapacitated and Katsuyu is still able to function just as it would if it were corporeal.

Katsuyu has also noted that its power is directly tied in to that of its summoner and their Strength of a Hundred Seal. For that reason, when Tsunade summoned the slug in her weakened condition during the Fourth Shinobi World War, the healing proceeded slowly, as not only was Katsuyu multi-tasking but Tsunade's reserves were extremely low.

Search for Tsunade
After Tsunade healed herself from what should have been a fatal attack, all three Sannin decided to use their most powerful summons and all three succeeded at the same time. Katsuyu was summoned and when the battle begun, it was constricted by Manda, but escaped using the Slug Great Division technique.

Guy and Lee Impersonator Arc
It was summoned again in episode 161, a filler episode, to deal with Mondai and Potcha, a pair of outsiders impersonating Might Guy and Rock Lee. In this instance it had a cold, and sneezed acid, which Mondai and Potcha, along with a clueless Naruto, were forced to anticipate and assist (although it is entirely possible that it was faking the cold in order to help Tsunade make Mondai and Potcha's lives miserable and prevent them from having the time to gather any information).

Three-Tails' Appearance
Fearing that Shizune's team would need further assistance to seal the Three-Tails, Tsunade sent Katsuyu to aid them. Four of its clones appeared in time to help the sealing team by giving them an extra supply of chakra. Ino and Hinata, who did not recognise Katsuyu, were initially alarmed, but Sakura explained who it was to them.

Six-Tails Unleashed
Tsunade sent Katsuyu to tell the Eight Man Squad about the Tsuchigumo clan for the mission.

Pain's Assault
When Pain began attacking civilians all over Konohagakure, Tsunade summoned Katsuyu. She then told the giant slug to take her chakra and heal the injured all over the village. When Pain used Shinra Tensei to destroy Konoha, Katsuyu surrounded all the villagers to protect them from the blast and heal their injuries. When Naruto returned to Konoha, Tsunade ordered one of its smaller clones to stay in Naruto's pocket and keep him informed of Pain's abilities. Other Katsuyu clones informed the villagers of the situation, and helped facilitate communication between them; Shikaku asked the clone nearest to him to have the other clones find out what they could from all the ninja who had fought Pain. After Hinata Hyūga was stabbed and Naruto lost control of himself to the Nine-Tailed Demon Fox, the clones on him observed this and reported the events to all the ninja in the village. The clones on Naruto survived his transformation, and told him that he had not harmed Hinata or any of the other villagers while under the Nine-Tails' control, which greatly relieved him. When Nagato proceeded to reincarnate those he had killed, the Katsuyu clone relays to Naruto the villagers coming back to life. After Pain's defeat, Katsuyu informed Sakura, Hinata (who had just regained consciousness) and Team Guy of Naruto's victory, and tried to tell Neji that Naruto did not want help, but Neji insisted that Katsuyu lead them to Naruto.

Fourth Shinobi World War: Confrontation
As Tsunade leaves headquarters with A, she apparently summons Katsuyu, who divides, with part of it going with Tsunade and part staying at headquarters to more than likely be used to keep the channel of communication between them open. When Tsunade and A confront Naruto and B, Katsuyu comments on the chakra of the Nine-Tails Chakra Mode. Katsuyu later expresses relief when Killer B intercepts the Raikage's attack on Naruto.

Meanwhile, back at headquarters, Katsuyu asks whether or not it would be best to alert the Raikage and Hokage of what was happening, but Shikaku states that he was appointed to handle things at headquarters while they tried to stop Naruto and B. When Katsuyu tries to inform him of what was happening with Tsunade and A, Shikaku asks for silence in order to concentrate of formulating a strategy to combat the disguised members of the White Zetsu Army that had infiltrated their ranks in the dead of night. Later, when Shikaku realises that the only way to expose the imposters was to use Naruto's ability to sense negative emotions that he gained through his control of the Nine-Tails, Mabui states that is impossible as the Raikage would not allow Naruto to participate in the war, Katsuyu informs them that this might not be entirely impossible due to the Raikage finally allowing Naruto and B to pass. Katsuyu later relates the information of what happened, much to Mabui's disbelief.

As Tsunade who had returned to headquarters prepares to depart for the front lines after hearing the reports of the real Madara Uchiha being revived and slaughtering the Fourth Division, Katsuyu calls out to her to stop. As Shikaku states that they should use the Flying Thunder God Technique to transport Katsuyu instead of Tsunade, Katsuyu said that would be fine since her body could withstand being ripped to shreds by Mabui's Heavenly Transfer Technique.

Fourth Shinobi World War: Climax
Katsuyu, in its entirety, is later summoned to Tsunade's side by the severely wounded Kage. Distressed about its master's condition, Katsuyu immediately states that it would put the bisected woman back together, but is cut off by Tsunade who tells the slug instead to first bring all the other Kage to her as there was still time to save them.

Later on, another part of Katsuyu was summoned by Sakura Haruno to the battlefield. Pleased that Sakura was finally able to complete the Strength of a Hundred Seal and exclaimed that Tsunade would be glad to hear this. Sakura, however, asked Katsuyu to leave that to less pressing times and requested that it split itself up and attach its clones to the members of the Allied Shinobi Forces, so that they could be healed. With this Katsuyu divided itself and made its way to the wounded shinobi and began transmitting the healing chakra.

Back with Tsunade, Katsuyu was shocked when Orochimaru, Suigetsu, and Karin arrived at their location. When the Sannin and his group made their way over to Tsunade, a wary Katsuyu prepared to attack them. The slug only stood down after considering Orochimaru's promise that he meant them no harm, and contemplating their current situation. When asked why the healing was progressing so slowly despite its presence, Katsuyu noted that due to Tsunade's weakened condition, it was not operating at full power as it had to maintain the summoning, while treating the other four Kage, and piecing Tsunade's body. Later keeping a watchful eye as the newcomers kept their word and healed Tsunade, Katsuyu later reported to her, and the other Kage about the situation on the battlefield.

Birth of the Ten-Tails' Jinchūriki
The clone of Katsuyu with Sakura later informed its summoner when the Version 1 mantle reappeared around her that it had sensed it while healing the others, noting that the shroud had never really disappeared entirely. When the Ten-Tails' tree form began attacking the shinobi, sapping chakra from their body, a worried Katsuyu looked on as Sakura lay on the ground. Though Sakura recovers from the ordeal and is informed by Katsuyu that the Kage were also on their way, Katsuyu later regretfully told Sakura that they could not remotely attend to Shikamaru because its clones had suffered the same fate as the shinobi who had come in contact with the tree leading Sakura to rush to her comrade's aid instead. When later Sakura is joined by Tsunade, the two summon one-tenth of the real Katsuyu to the battlefield. When summoned, Katsuyu immediately deconstruct underneath the shinobi creating a literal recovery area where the shinobi would be able to heal themselves and continue fighting.

Trivia

 * The name Katsuyu apparently comes from in on'yomi.
 * Katsuyu's real size which is presumably very enormous and is a combined form of all the body-parts, had never been seen in the series.
 * The first part of Katsuyu's name:, means "split" or "divide". This could be a reference to its ability to divide into many smaller bodies.
 * A variation of the worldwide game "Rock, Paper, Scissors" in Japan presented in the early 1640s was the "Slug, Snake, Frog" variation. In this variation, the Frog is afraid of the Snake, the Snake is afraid of the Slug, and the Slug is afraid of the Frog. This is influenced by the Sansukumi way of thinking where it keeps the three objects in a deadlock. This concept was referenced and joked upon in an omake of episode 164 of Naruto: Shippūden, where Katsuyu feared that with Manda deceased, Gamabunta and the toads of Mount Myōboku would now try eating it. This was also played for in episode 8 of Rock Lee no Seishun Full Power Ninden where after Tsunade and Naruto summoned Katsuyu and Gamabunta, the latter swallowed the former shortly after appearing, much to Tsunade's horror.
 * In English translations of the series, Katsuyu is referred to with female pronouns. In the original Japanese, Katsuyu is only referred to with gender-neutral pronouns which could relate to slugs being naturally hermaphroditic.