Katsuyu

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

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, uses Mistress, or Milady.

Search for Tsunade
After Tsunade heals herself from what should have been a fatal attack, all three Sannin decide to use their most powerful summons and all three succeed at the same time. Katsuyu is summoned and was nearly squeezed to death by Manda, escaping by using her Slug Great Division.

Mondai and Poccha
She is summoned again in episode 161, a filler episode, to mess with Mondai and Poccha, a pair of outsiders impersonating Might Guy and Rock Lee. In this instance, she had a cold and sneezed acid which Mondai and Poccha, 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 Poccha's lives miserable and prevent them from having the time to gather any information).

Three-Tails Arrival filler arc
Fearing that Shizune's team would need further assistance to seal the Three-Tailed Giant Turtle, Tsunade sent Katsuyu to aid them.

Pain Invasion arc
When Pain began attacking civilians all over the Leaf Village Tsunade summoned Katsuyu. She then told the Giant slug to take her chakra and heal the injured all over the village. When Pain uses Shinra Tensei to destroy Konoha, Katsuyu surrounds all the villagers to protect them from the blast and heal their injuries. When Naruto returns to Konoha, Tsunade orders one of her small divisions to stay in Naruto's pocket and inform him of Pain's abilites. Other Katsuyu divisions inform the villagers of the situation and help facilitate communication between them; Shikaku asks the division nearest to him to have the other divisions find out what they can from all the ninja who fought Pain. After Hinata Hyuga is stabbed and Naruto loses control of himself to the Nine-tailed Fox, the division on him observes this and reports the events to all the ninja in the village. The division on Naruto survives his transformation, and tells him that he did not harm Hinata or any of the other villagers while under the fox's control, greatly relieving him.

Abilities
Compared to Gamabunta and Manda, Katsuyu is rather slow yet quite 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.