Katsuyu

is the slug queen summoned by Tsunade. The name Katsuyu apparently comes from 蛞蝓, or slug in kanji, read 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 as Mistress, or Lady.

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

Anime Fillers
She is summoned again in episode 161, a filler episode, to mess with a pair of outsiders impersonating Might Guy and Rock Lee. In this instance, she had a cold and spat acid which the two impersonators, 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 the intruders lives miserable).

Pain Invasion Arc
When Pain began attacking civilians all over Konohagakure Tsunade summoned Katsuyu. She then told the Giant the slug to take her chakra and heal the injured all over the village

Abilities
Compared to Gamabunta and Manda, Katsuyu is rather slow yet quite subtle. With her, 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. She can also spit acid for her, which is strong enough to melt through anything with ease.