Katsuyu

is the slug queen/king 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 as 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).

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, and takes Tsunade to safety as Naruto fights pain.

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 her/himself 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.It has been revealed that information could be spread through the multiple bodies of Katsuyu making her perfect for spying missions. She can also spit acid for her Tongue Tooth Sticky Acid, which is strong enough to melt through rock with ease.