Katsuyu is the giant Slug Queen summoned by the Fifth Hokage, Tsunade. She is powerful in all areas including defensive and offensive capabilities. She is summoned only in rare times of great need, such as when Pain invaded Konoha.
The history of Katsuyu is unkown, as there is very little about her past and not that much in regard to her at all. What we do know is she must be somewhat aged, as she is just as old as Tsunade. We can tell this because she has always been the main summon for her from the beginning, as well as Gamabunta being able to recognize her when they were all summoned to fight. She also shows sign of being aged because she is referred to as the "Slug Queen" meaning she had to work for the title, as well as be old enough to take the position.
Katsuyu was created and illustrated by Masashi Kishimoto, there is currently no knowledge of any inspiration given for the creation of this character. Katsuyu first appeared in Naruto: Episode # 95, and first appeared in the manga in Naruto: # 19. Katsuyu appeared when many of the other summons appeared for the Sannin, such as Gamabunta and Manda.
Katsuyu also appears right when her summoner Tsunade, makes some of her first appearences in the series. This could be an indication of how close the two's relationship is. Tsunade only appeared two books before Katsuyu in the manga, and ten episodes in the anime. Katsuyu can, in some languages, mean "slug." It is also interesting that the old Chinese hand game, "Slug, Toad, Snake" are the same animals that the Sannin use for their main summons. This may be a "hint" from the Masashi that the Sannin are more or less equal in power, because in the game it is known that the Slug fears the Toad, the Toad fears the Snake, and the Snake fears the Slug. This game is very similar to the American game of, "Rock, Paper, Scissors" and may be where it gets its roots.
While all three of the Sannin, Tsunade, Jiraiya, and Orochimaru were battling, they all summoned creatures at once. After they were summoned, Katsuyu was the first to attack, by using her Sticky Acid technique. She shot this acid right at Manda, but he was already dodging the attack, so it missed him and hit the rock he was coiling around. We see the acid burn through the boulder effortlessly. Then, we see Manda blitz Katsuyu, and coil around her, pressuring her to bust. With Tsunade's command, Katsuyu uses her Slug Great Division technique to break into thousands of tiny slugs, breaking out of Manda's grip only to rebuild behind him.
While Katsuyu was using this ninjutsu, Gamabunta was attempting to stab Manda through the head with his dagger, but failed to do so when Manda just chomped down on the sword, then there was a tugging battle between who would get it. When Katsuyu had finally come back together, Manda quickly yanked on the dagger, pulling it out of Gamabunta's grip and by swinging his head threw it straight towards the toad summon. However, with his powerful hind legs, Gamabunta jumped sideways and out of the path of the dagger, only for it to land right in front of Katsuyu. Being a perfect oppurtunity, Tsunade uses this to jump and hop back up on Katsuyu's head. Afterwords, Jiraiya tells Tsunade and Katsuyu to stay back while he attempts to burn Manda, Orochimaru, and Kabuto. Only to find that they were underground, and that they nearly killed them by coming up from behind with an attack.
Luckily, Tsunade was able to use Gamabunta's gigantic dagger to drive in through Manda's head, preventing any movement of the head at all. Shortly afterwords, all of the summons were released, the only remark made by any of them was Manda to Orochimaru, warning him of the trouble he would get the next time they met.
After Pain had come to Konoha, he began causing mass destruction and injuring many of the villagers. Because of this, Tsunade knew she had to help her people heal, so she summoned Katsuyu to help her do so. In order for Tsunade to be
able to heal the entire village, Katsuyu would have to use her Slug Great Division technique. She would tavel all over the city, and use Tsunade's chakra to heal all the injured, by touching there wounds then transferring the chakra to their cells, allowing them to heal much faster than normal, as well as saving many lives, or at least making them longer until medical-nin could arrive to give further and more invasive care.
The smaller Katsuyu's have the ability to telepathically communicate with each other, Tsunade, and talk. With these abilities, they kept Tsunade informed of the entire village, from corner to corner. This also allowed the villagers to be informed on what was going on, and this would also allow them to be informed on the latest knowledge, from Tsunade herself. Being the source, Tsunade would be able to regulate what the villagers could know, so this was also a positive note for her. Katsuyu also protected villagers from direct harm, when Pain used Almighty Push against Konoha, Katsuyu covered as many as she could from the direct blast.
Sticky Acid SpitIs a technique that is used by Katsuyu in battle, and is the only pure offensive attack she is credited to
have. This attack is normally shot within mid-range of Katsuyu and her attacker, and only a small-to-medium amount is spit from her mouth so that she can conserve and use as much as possible in the long-run. It is a yellow and light brown color, very similar to mud or dirt. When it hits an object or Katsuyu's target, it begins to breakdown the substances that make up that object and it starts to "burn" through it, or so it appears to. All in all, it takes about two-to-three whole seconds for the acid to degenerate whatever it touches, and nothing has been shown or said that could stop the acid from working. Katsuyu's aim is decent for the spit, but she has only used it majorly once and that was against the very fast and agile Manda, the perfect opponent to dodge her offensive capabilities. Therefore, it hit the boulder near him, and not actually the smake itself.
Slug Great Division
More of a defensive or helpful technique, this is the jutsu shown most often in Katsuyu's appearences. It is uncertain exactly how many smaller versions of Katsuyu are made with this technique, but it can be assumed many thousand considering her size when in her standard state. Also supporting the great number of smaller Katsuyu's is the shear number of people she protected and healed using Tsunade's chakra during the Invasion Of Pain Arc. This technique also makes her very "durable" against any type of physical attack, because as soon as she is hit, she can just burst into many smaller versions, then rebuild back again.
This technique is purely supportive and helpful technique. This was only showcased a few times, but its effect was much greater than that. This technique was the reason why the
many small Katsuyu's were able to help many Konoha villagers with there medical needs when Pain invaded. They would begin to glow a greenish blue color when they used Tsunade's chakra, though they had to be very careful not to use any more than needed for the healing process due to pooling from only one person's chakra. It is also unkown how many lives Katsuyu saved directly, though it can be assumed it was many of Konoha's population due to the amount of damage caused by Pain's invasion. It's possible that this technique also gives Katsuyu and Tsunade the telepathic connection they have, because they are using the same chakra, though this isn't certain and it has not been confirmed.