#byakugo

Katsuyu, also known as Katsuyu of the Shikkotsu Forest, is a giant slug from the Shikkotsu Forest, a secret and legendary place from the Three Unexplored Sage Regions (仙人三大秘境, Sennin Sandai Hikyō). 

The Three Sage Unexplored Regions according to the Fourth databook.

She is the personal summon of the Fifth Hokage Tsunade, and later, her disciple Sakura Haruno; both women are probably the only people in the world that have been able to summon the slug.

Katsuyu possesses one of the most unique qualities and abilities among animal summons, most of which are still a mystery within the series. 

— WHAT IS A SUMMON AND ANIMAL SUMMONS?

A summon is the ability to invoke objects, weapons, people or animals. This action is carried out through a Summoning Technique (口寄せの術­, Kuchiyose no Jutsu), a special hand seal based jutsu to transport any element to the user’s location for a limited amount of time. 

A Summoning Technique is classified as a space-time ninjutsu and a C-Rank (chūnin level) difficulty jutsu. 

Summoning Technique according to the First databook.

Usually, in order to summon objects or weapons, users apply scrolls or special sealing techniques to contain the desired material. This is the case of Tenten, for example; she uses scrolls to seal multiple kunai, shuriken and any weapon of her choice, granting her the title of “Master of space-time ninjutsu”.

However, for animals and living creatures, this is different.

An animal summon is a technique that adds to the ninja’s ninjutsu repertoire, as a shinobi can take advantage of the animal’s abilities for battle or other purposes.

Before any animal can be summoned, the animal and the user must sign a contract with the species or the specific creature using their blood. In order to summon said animal, the user applies some of their blood, normally by biting their fingers, and channels their chakra to their hands thus invoking a sealing pattern that transports their summon to that same location. The amount of chakra used to summon the creatures determines the size and number of summons. 

The summoning contract of the Toads of Mount Myōboku, Naruto chapter 92.

In the case of the Three Unexplored Sage Regions, the summoning contracts vary. 

Naruto Uzumaki Jiraiya and Minato Namikaze sealed a contract with the toads of Mount Myōboku using their own blood, so when they perform a Summoning Technique, any available toad can offer their services to them. 

Orochimaru has his contract with the snakes of the Ryūchi Cave tattooed on his arms. The methods his disciple Sasuke Uchiha uses to summon snakes from the same location is unknown. 

Tsunade and Sakura Haruno do not have a physical contract, or at least it has never been revealed. However, by reading the manga, the contract with the Shikkotsu Forest might be through their Strength of a Hundred Seal, as it is only possible to summon Katsuyu if the summoner possesses this seal.

It is unknown if there is a blood contract involved, but it is likely that there is since the method of summoning is blood like his companions. 

Orochimaru's contract Naruto chapters 165 and 344.

— KATSUYU AND HER ABILITIES, WHAT MAKES HER SPECIAL?

Katsuyu is a very intriguing and interesting animal and the only one whose past has never been explored.

Summon’s habitats and the Three Unexplored Sage Regions are home to a single species of animals. 

The Ryūchi Cave is home to snakes such as the White Snake, Manda or Aoda, and Mount Myōboku is home to toads like Gamakichi, Gamamaru or Gamabunta.

But, unlike those, Katsuyu is the only organism living in the Shikkotsu Forest. She does not have any other slug companions. 

Following the same scheme as the rest of summons, Katsuyu resembles a real slug. 

She is white or ivory in color, with three turquoise stripes running on her head and sides that start on her head and end in her tail. 

One special feature about her appearance is that her true size has never been seen. Using one summoner, Katsuyu is bigger than the Hokage Residence, and using two summoners, Katsuyu is the size of the roots of the God Tree, which is just one tenth of her real size according to Tsunade. 

Katsuyu's size summoned by one person or two, Naruto chapters 422 and 650.

Her size changes depending on the amount of chakra used to summon her. She can appear as a small slug used to gather intel or help in supplementary tasks, or she can appear as a giant creature using one or two summoners. 

Small Katsuyu summoned to assist Headquarters Naruto chapter 561.

Her age is a true mystery. It is known that toads and snakes live many years and have descendants, such as Gamabunta and Gamakichi’s father and son relationship, or Aoda having great grandchildren; or certain summons retiring when they reach an age while their companions or children replace them. 

The Three Unexplored Sage Regions are legendary places known for many generations. 

Katsuyu, on the other hand, is the only inhabitant of the Shikkotsu Forest. She does not have descendants or companions in her home habitat. Her age or her true origins are a mystery. The first shinobi to discover her existence or previous masters before Sakura Haruno and Tsunade are also unknown.

The first and most important thing to point out is that Katsuyu can only be summoned by users of the Strength of a Hundred Seal. This seal is an extremely difficult jutsu to learn and achieve, so there have only been two summoners in history.

The reason for this is that Katsuyu is such an enormous animal that it requires an immense amount of chakra to summon, and that chakra is stored inside the seal. 

Tsunade and her disciple Sakura Haruno are the only beings able to summon Katsuyu Naruto chapter 650.

The primary skill Katsuyu has is the ability to divide herself in many fragments, hundreds or thousands of them in fact. This technique is called Slug Great Division (蛞蝓大分裂­, Katsuyu Daibunretsu) and it can be used for supplementary purposes, as well for battle. 

In a fight, thanks to her rapid ability to transform her body and divide herself, she can separate herself to nullify attacks by avoiding any strike at incredible speeds. She can do this as many times as she wants without exhaustion. Each of these fragments are also quite fast, able to throw themselves in front of people to shield them from danger.

Manda trapping Katsuyu but she escapes Naruto chapter 170 and Second databook.

She also has the ability to spit a sticky and elastic acid, a jutsu called Tongue Tooth Sticky Acid able to melt stones and vaporize everything it touches (probably senjutsu although never truly revealed). It is also very difficult to dodge due to its unpredictable nature. 

Slug acid, Naruto chapter 170 and Second databook.

Unlike Shadow Clones or other regular clones that are independent from each other and are not interconnected, what Katsuyu does is divide her own body in equal parts, so she is in as many places as she wants at once. She is one entity divided into many other independent and interconnected parts while remaining one single body. Each of her divisions behaves like their own person, so she can complete different tasks while also knowing what her other parts are doing in other places.

Being the summon of medical ninjas, Sakura Haruno and Tsunade take advantage of Katsuyu’s ability to divide herself in order to channel their healing chakra and treat as many people as necessary. 

With a technique called Immense Network Healing (蛞蝓・網療治夥­, Katsuyu: Mōryōjika), Katsuyu collects chakra from the Strength of a Hundred Seal to transform it into healing chakra and treat the injured. 

Using the Great Division and Immense Network Healing in tandem, Katsuyu has the ability to heal and protect everyone she makes contact with. Healing any kind of injury or absorbing people inside her in order to shield them from attacks or sudden explosions is her specialty. Her body is able to resist explosions and even Kurama's corrosive chakra.

Katsuyu divides her body and heals the Alliance Naruto chapter 634 and Fourth databook.

In fact, Katsuyu's power is not only limited to healing wounds, but regeneration too. One of her most impressive feats is the ability to maintain a patient in suspended animation, keeping them alive for as long as she possesses chakra from the seal in order to ensure their survival. 

Katsuyu was able to keep Tsunade alive despite having her body bisected, which on normal circumstances, would mean instant death. However, the slug's regenerative abilities allowed Tsunade to remain alive until she obtained a chakra boost from a third party. This fact is known as Tsunade does not really present the unique Uzumaki Clan ability to keep themselves alive for as long as possible.

Katsuyu kept Tsunade alive despite having her body cut in half, an impressive feat Naruto chapter 635.

This jutsu also acts as a sensing technique due to Katsuyu being connected to the chakra of the injured in order to heal them and her master. Katsuyu is able to relay Kakashi Hatake and Chōza Akimichi’s conditions to Tsunade through their connection, as the slug is connected to the Strength of a Hundred Seal; as well as being able to tell Sakura that Kurama’s chakra was still connected to the Allied Shinobi Forces.

Katsuyu can transmit the chakra levels and the condition of everyone she is in contact with Naruto chapter 427.

This way, Katsuyu was able to protect Konoha’s citizens from Pain’s Shinra Tensei and even survive Kurama’s toxic chakra. 

Able to be in many places at once, her ability to cover infinite distances while her parts remain interconnected allows her to fulfill communication and intel gathering roles. Being in many places and her clones being part of the same body, Katsuyu relays information to one person or multiple people, something she could do by attaching herself to someone’s shoulder and speaking. 

It was Katsuyu the one giving Naruto Uzumaki information about Pain and also told him that Nagato had revived Konoha citizens through the clones that stayed in the village, kept every villager informed about the situation and followed evacuation protocols when Naruto transformed into six tails, and told Tsunade everything that was happening in the battlefield thanks to having another body summoned by Sakura on the other side of the country, and vice versa.

Sakura instructing Katsuyu to evacuate the area through the multiple clones Naruto chapter 438.

This communication ability can cover immense ground at once because the communication is instant. It works similar to regular telepathy, but instead of hearing the other person's voice in their head, people are talking to Katsuyu, and Katsuyu in turn is actually talking to herself through her clones.

Katsuyu's clones on different parts of the Land of Lightning tell Tsunade and Sakura the events that have taken place on both ends Naruto chapters 635 and 647.

An animal summon can only perform at the top of their abilities for a limited amount of time, what is known as the summoning time limit. 

It is unknown exactly how long the invocation lasts, or whether it varies between animals. A summon will help its master before the jutsu runs out or the summon has exhausted its power. 

Gamakichi mentions the summoning time limit Naruto chapter 642.

However, for Katsuyu, this is not the case. 

The slug can be summoned for unlimited periods of time and several times a day, as seen when Katsuyu spent the Fourth Shinobi World War by Tsunade’s side in the Headquarters, primarily helping Shikaku Nara and participating in the planification of the troops, and later with the Five Kage against Madara and Sakura with the Allied Shinobi Forces.

This could be explained by the symbiotic bond Sakura and Tsunade share with Katsuyu through the Strength of a Hundred Seal. 

The seal stores unimaginable quantities of chakra inside, which are used by Katsuyu to perform healing on people. As long as Katsuyu has been transmitted every drop of chakra in the seal, she will continue her work, as seen when Tsunade gave the slug all the chakra inside her Byakugou to the point it disappeared but Katsuyu physically remained in Konoha until her services were no longer needed.

Tsunade's Byakugou disappears but Katsuyu still serves her purpose as a summon to evacuate Naruto chapter 430.

Still, like any other summon, Katsuyu can be exhausted from battle and disappear. When the Ten Tails absorbed the chakra of every shinobi in the battlefield as the Kurama Chakra Cloak, every Katsuyu fragment attached to them was also absorbed and vanished, but the main body remained intact protecting Sakura Haruno form the tree. 

Katsuyu's clones disappeared absorbed by the God Tree Naruto chapter 647.

— THE PERFECT SUMMON?

Katsuyu is very different from the rest of summons. 

From the immeasurable amount of chakra it requires to summon her, to her multiple multitasking and supplementary abilities and her mysterious origins, Katsuyu might be the perfect summon and the strongest animal summon of Naruto and Boruto.

Her ability to divide herself in thousands of fragments, a body and chemical composition resistant to blasts that can reshape her own state, no summoning time limit, outstanding healing and regeneration factor, the unlimited wide range communication she offers, acidic attacks, useful to gather intel by herself and turn small enough to offer battle assistant, and a sensory connection with her master and other people she is attached to. 

Katsuyu is the ultimate creature.

Without a doubt, the most essential ally in desperate times. A valuable help in case a village is being attacked by powerful enemies that threaten to destroy everything around them, Katsuyu is the only weapon able to protect thousands of people from any attack and offer assistance while in contact with her master and everyone she is with.

Thanks to her, every Konoha villager that had not been killed by the Pain Paths previously could survive the Shinra Tensei explosion that destroyed Konoha, which later allowed the surviving shinobi to handle evacuations, as well as allow Naruto to obtain intel about Pain and Nagato or be told that Konoha's villagers were revived.

Katsuyu is an unrivalled intel summon Naruto chapters 431 and 449.

She was also the main link of communication between the Headquarters and the two different fronts of the war, Five Kage and Allied Shinobi Forces, and the reason why the Five Kage survived the fight despite their fatal injuries.

Lastly, it was also Katsuyu who protected the Allied Shinobi Forces against the roots of the God Tree, saving a large portion of the shinobi from death, and later offered a healing area for their future fight against the Kanzeon Lotus King.

— CURRENT WHEREABOUTS.

Ever since the death of Madara Uchiha, Katsuyu has not been summoned again.

With no threats to the destruction of Konoha or the need to establish connections between battlefields, Katsuyu was not summoned in the Blank Period. 

In Boruto, Katsuyu’s debut is strictly linked to Sakura’s appearances: if Sakura does not appear, neither does the slug. This is something that is hard to fathom considering that the village has been exposed to explosions, invasions and multiple deceased shinobi ever since the start that could be prevented by Katsuyu like previous times.

Katsuyu appeared in the anime arc “Kawaki and Himawari in the Academy” where the students had to put on a play about the Sannin and Kawaki dressed as Katsuyu.

Kawaki representing Katsuyu in Boruto episode 267.

There’s no mention of any seal. Nothing. We only know the following from the manga about her:

One) She knew “Threat Perception” like KCM Naruto (which is how he managed to detect Kisame who was hiding inside Samehada):

Two) She had special chakra like Kushina that was more powerful than her clansmen possessed, which is why she was selected as a Jinchuriki for Leaf:

Three) She offered her services to seal the Nine-Tails inside her to aid her husband in his reign fascism:

Four) She convinced Kushina to spread a fascist village’s dogma through love, or something, and carry on the barbaric tradition:

Five) She’s remembered by Kurama (when he thought of his own imprisonment at the hands of Leaf Shinobi and Uchiha Madara) and Madara:

That’s it. There’s no other mention of Mito beyond this. We know that she’d special chakra in her clan like Kushina; she sealed Kurama inside herself through a Fuinjutsu Seal; and she had Threat Preception like KCM Naruto. Beyond that, there’s literally no mention of Mito in the entire manga. She’s got literally a single dialogue that she delivered to Kurama in the entire manga. The small bit I posted above.

Did she pass on anything to Kushina other than sugar-coated fascism, seals, and pep-talk to carry on the “Weapon of Mass Destruction” rhetoric? No. This is Uzumaki Kushina’s Databook Profile:

The manga makes no mention of anything beyond the scans I’ve shared that show the interactions between Kushina and Mito. What about the Byakugo itself? This is what the Databook 4 states about the Byakugo:

It’s a mix of Strength of a Hundred Seals and Ninja Art: Creation Rebirth (which she used in Part I against Orochimaru). What are Strength of a Hundred Seals and Creation Rebirth? This is what Databook 2 States on the two:

Yin Seal: Release

(Infuuin: Kai)

Fuuinjutsu, S-rank, Supplementary

User: Tsunade

Unravelling the key to using the Creation Rebirth!!Creation Rebirth, the ultimate medical technique. However, using this technique requires such an enormous amount of chakra, that it is beyond any shinobi to mould it in an instant. Tsunade, using the chakra stock kept in the seal on her forehead, has cleared this prerequisite. With Yin Seal: Release, the seal is released, becoming the source for the body regeneration technique.The moment it is unsealed, a pattern appears on the forehead.

Releasing the seal is the greatest danger signal.

Creation Rebirth (Souzou Saisei)

Ninjutsu, S-rank, Supplementary

User: Tsunade

The absolute pinnacle of medical ninjutsu~~ a forbidden regeneration technique!

Created by the greatest of medical ninja, Tsunade, this is the ultimate regeneration technique!! By releasing a large amount of chakra at once, the body’s cell division is forcibly stimulated, reconstruction all organs and all tissues making up the human body!

If this technique is used, a body whose vital organs are so gravely injured that it cannot bear it any longer will be instantly restored to its uninjured state. It’s impossible to die by any means… for the sake of continuing to protect the lives of one’s comrades. That’s the resolution hidden in this technique.

By releasing the great volume of chakra stored in her forehead, Tsunade can make her body do the work of an untold number of years in an instant.

This wound may have brought her on the verge of death, but she won’t die~~

The acceleration of cell division that allows for the complete recovery from wounds is paid for with the shortening of one’s life span.

The former is what you see on Tsunade and Sakura’s forehead. It’s a compensation for the lack of chakra-volume Tsunade has on her that Hashirama naturally had in spades. That’s the reason why Madara mocked her:

So, basically, it’s a compensation for the lack of chakra and skill compared to Hashirama who used to manage a colossal amount of chakra compared to her and healed himself with the greatest effortlessness (really, her chakra-control, like her student’s, isn’t anything special as the chakra inside the seal is regulated or kneaded chakra that was gathered through years of focus, not weeks or months). Heck, he had more chakra than BM Naruto did, combined with the amount he passed on to the surviving members of the Shinobi Alliance:

Her chakra reserves are nothing compared to Uchiha or Hashirama’s. Sakura’s got abysmal chakra reserves, too. This is precisely the reason why she managed to climb the tree so quickly while Sasuke and Naruto couldn’t. It had nothing to do with talent. Large chakra reserves are far harder and trickier to manage and control (Madara blew up and turned into Kaguya for the same reason):

It took Sakura three years to fill up the seal (prodigy, my ass!), so I can assume that it took Tsunade the same time, too. Again, “chakra compensation” as the seal is a knock-off of Hashirama’s ability to heal continuously and without seals:

Another important thing is the Byakugo itself. If you look closely at the Databook 4 page I posted, it can’t heal from very grave wounds. That’s the reason why Tsunade was stabbed by the real Madara (577 pg. 11):

He was basically carrying her around like a kebab on a stick, and she was completely paralyzed. She literally couldn’t move when she the seal was not only active, but she’d also released it not long before this and had used it to heal nothing (577 pg. 12):

In the next chapter, she breaks the sword and uses it against Madara. Again, she’s at full strength here and completely fresh, and she ... couldn’t even crack the real Madara’s Susanoo (578 pg. 4):

578 pg. 5:

578 pg. 6:

As you can see: no cracks, no damage, and no aura dispersal. It’s kind of ineffective as a Healing Jutsu, too, but I digress as my point was to show that the forehead seal isn’t anything remarkable compared to the healing Hashirama used. It didn’t lower his life-span, either. Even Karin’s healing is far more impressive by comparison: she bites her flesh and her chakra heals automatically. No seals. No waiting. No recharging.

Tsunade’s forehead seal could be inspired from Mito’s in regard to its design only, but it doesn’t work like hers. It only looks like hers, so the similarity is quite superficial. We don’t know what Mito even had on her forehead. Tsunade’s seal is designed to gather chakra and heal; so in that regard it’s modeled after Hashirama’s ability to heal with absolute finesse. It’s a compensation for Tsunade’s mediocre reserves and Sakura’s poorer ones. Nothing more. Nothing less. Nothing in the manga even remotely hints at anything of the sort.

Mito didn’t pass on her seal to Tsunade. Her forehead seal isn’t “Strength of a Hundred Seals”. She didn’t use anything like Byakugo. In fact, she shared a similar ability of “Threat Perception” with Naruto; since, she’d have purified Kurama’s chakra to do that, she’d have a much better healing than the Byakugo, anyway, as Kurama’s purified chakra is so full of Life Energy that it was capable of growing fresh branches and leaves out of Yamato’s discount-variety Mokuton (to call his Mokuton pathetic compared to Hashirama’s would be the grossest understatement):

Why would she need healing, anyway, when she’s got this at her disposal? And Boruto Manga? You mean these scans?

That doesn’t negate what I’ve said as it doesn’t mention anything about Mito or what the seal stood for at that time, but it’s interesting when these exist in Canon:

The Sage era was so long ago that he was considered a myth! Now, the Jutsu, apparently, existed since his time. Oh, boy, what the heck? If Kakashi didn’t know about the Sage, how would Tsunade? Remember, she left way back after the second war and never returned. While younger than her, Kakashi’s served in active duty as a Jounin since before he stepped into his teenage years. Even Jiraiya didn’t know much about the Sage beyond myth and prophecy:

The Child of Prophecy has nothing to do with the Sage, either. How on earth did Boruto Manga reach to this conclusion when the Sage was literally considered a part of folktale, a myth? Why would the Sage even need to store chakra when he’s the child of Kaguya (Juubi and Shinju combined)? Why would any one of his direct descendants need to store chakra when half of his chakra alone brought Naruto and Sasuke almost on equal footing with Sage of the Six Paths’ Madara (granted he was missing one half of Kurama and a bit of Gyuuki; Obito took a big chunk of tailed-beasts’ chakra from him, as well, and gave it to Naruto; but you get the idea)? How did Tsunade find out about this when she’s the least knowledgeable on this front and when no one had any idea about the Sage himself beyond the fact that he had the Samsara eye, Rinnegan? They didn’t even know anything about the Rinnegan’s abilities! This is laughable.

Anyhow, they’re trying to grant some depth to the Yin Seal, nothing more. The seal itself, as I mentioned, looks like Mito’s seal, but it isn’t designed to work that way: it’s modeled after Hashirama’s healing ability. There’s nothing more on this front in Naruto canon or Boruto’s from the looks of it.