Katsuyu

is the slug queen 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. She is predominantly white in color with a blue streak that covers most of her top half running down from her head and tapering off at her tail. Her optical tentacles have a slight tint of gray to them, she also has two sensory tentacles on either side of her mouth.

Personality
Compared to Gamabunta and Manda, the summons of Jiraiya and Orochimaru respectively, Katsuyu is quiet and more subservient to her master. She 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 her considerable respect, addressing her as "Katsuyu-sama." In the English dub, she is referred to as "Lady Katsuyu."

Abilities
Compared to Gamabunta and Manda, Katsuyu is rather subtle. With her Slug Great Division, Katsuyu can divide herself into smaller slugs to evade attacks or to serve small scale functions, and is able to quickly reform herself if need be. These smaller slugs are also able to attach themselves to injured people, allowing Tsunade to channel healing chakra through Katsuyu to the injured person. Unlike other types of clones, though, Katsuyu's divisions can share information without having to be dispelled. She can also spit acid for her Tongue Tooth Sticky Acid, which is strong enough to melt through rock with ease.

Furthermore, Katsuyu has been shown to be extremely resistant to deadly attacks, as seen during Invasion of Pain Arc,not just when her smaller clones took the power of a large-scale Shinra Tensei head on, but also when a palm-sized version of herself survived Nagato's Chibaku Tensei and the corrosive chakra of Naruto's eight-tailed form.

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 her Slug Great Division.

Guy and Lee Impersonator Arc
She 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 she 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 she 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 divisions of her 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 she 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 her 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 "scissors, paper, stone" 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 her.
 * Katsuyu is also shown to have a playful side and a jovial friendship with Tsunade as seen in the omake at the end of Shippuden episode 118. She tells Tsunade of the way Sakura overreacted when she fabricated a story of her difficulty journey (including: the physical limitations of her 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.