Katsuyu

is the slug summoned by Tsunade. The name Katsuyu apparently comes from in on'yomi.

Appearance
Katsuyu is very large in stature and is easily as tall as the largest building in Konoha. It is predominantly white in colour with a blue streak that covers most of its top half running down from its head and tapering off at its tail. Its optical tentacles have a slight tint of grey to them, it also has two sensory tentacles on either side of its mouth.

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 honorific "-sama", and, in the English versions, calls her Mistress or Milady. Katsuyu tends to also use honorifics 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".

Abilities
Compared to Gamabunta and Manda, Katsuyu's abilities are rather subtle. 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 slugs to evade attacks or to serve small scale functions, and is capable of quickly reforming itself if need be. The smaller slugs can attach themselves to injured people, allowing Tsunade to channel its chakra through Katsuyu in order to heal the injured person. Unlike other types of clones, Katsuyu's divisions can telepathically share information with others without having to be dispelled.

Furthermore, Katsuyu's clones have been shown to be extremely resistant to high amounts of damage, as seen during Invasion of Pain Arc. The clones were capable of absorbing people into their slime-like bodies, thus protecting them from physical impacts like the large scale Shinra Tensei. 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 divisions were fast enough to throw themselves in front of Shizune, Ino and Inoichi before Animal Path's rhino summon managed to hit them.

Search for Tsunade Arc
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 was nearly constricted to death by Manda, but managed to escape by using its Slug Great Division.

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

Tsuchigumo Kinjutsu Arc
Tsunade sent Katsuyu to tell the Eight Man Squad about the Tsuchigumo clan for the mission.

Invasion of Pain Arc
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 small divisions to stay in Naruto's pocket and keep him informed of Pain's abilities. Other Katsuyu divisions informed the villagers of the situation, and helped facilitate communication between them; Shikaku asked the division nearest to him to have the other divisions 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 division on him observed this and reported the events to all the ninja in the village. The division 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. 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.

Trivia

 * A variation of the world wide 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.
 * Katsuyu is also shown to have a playful side and a jovial friendship with Tsunade as seen in the omake at the end of Shippūden episode 118. It tells Tsunade of the way Sakura overreacted when it fabricated a story of its difficulty journey (including: the physical limitations of its slug body, falling off a rope bridge, and being attacked by villagers) in reaching Team 3 during the Three Tails Arc. Tsunade comments on Sakura's naivete and they both share a chuckle at Sakura's expense.