"The answer is Tobi!" muttered Kuroto in realization, and added, "Tobi can help Karin."
As for how will Tobi help Karin? The answer is pretty simple, just think of the time Obito relied on Tobi.
After being rescued and healed by Madara, Obito wasn't in any condition to help out Kakashi and Rin, he may have wanted to immediately rush to them, but considering his physical condition, let alone helping them, even reaching there would have been impossible for Obito.
At that time, a special Zetsu avatar, Tobi helped Obito.
Tobi covered Obito's body like armor and provided him with the Chakra supply. With the help of that chakra supply, not only Obito was able to break through the cement wall, he was able to bear the evolution of his Sharingan into Mangekyou Sharingan and cause the annihilation he performed after that.
Inspired by Tobi, Kuroto came up with the idea of creating a similar armor for Karin that would be made up of Hashirama Cells.
Since Karin needs to put all her attention into the fusion of the pair of Eternal Mangekyou Sharingan, she wouldn't be able to survive the Hashirama Cell implantation, therefore, something like Tobi would be very helpful for her. With the assistance of an external armor made up of Shodaime Hokage's cells, the burden on Karin will be reduced and she will be able to take a breather.
Moreover, the supply of Yang Chakra from the armor would also improve the success rate of the evolution of the two pairs of Eternal Mangekyou Sharingan into the Rinnegan, therefore, the armor is the best choice.
With this in mind, Kuroto gently caressed Karin's head, and said, "Bear with the pain for a while, I know what to do in order to ease it, however, it will take some time."
Karin gritted her teeth and said with a smile, "I can hold on, no worries Kuroto-sama!"
Kuroto nodded and went to the Cell Cultivation Room.
Here, he took out the Hashirama Cells, as well as Tobi's cells from the cold storage and started to work on them.
The idea of creating such an armor made up of Hashirama Cells isn't new, he thought about it a long time ago, that's why he has always kept Tobi's cells with him and made a lot of plans on how he would make the armor, the reason he never actually made the armor is that he is always too busy with something or the other and doesn't have enough time to bring that idea to life.
Now that there is a need for that armor, Kuroto can finally create it.
After working on the cells, Kuroto had all the data before him, and using that data Kuroto first drew a rough sketch of how the armor should be.
Unlike Madara, Kuroto doesn't need the armor to have any consciousness of its own, providing that won't be too difficult for him, however, there is no need. What he needs from the armor is to have the ability to be able to supply chakra to Karin, have a strong defense, should augment her strength, and should not affect her mobility.
"The armor should be made up of Shodaime Hokage-sama's cells for the supply of Yang chakra… however, it is an undeniable fact that if only those cells are used, keeping the armor stable would be very difficult. They are too aggressive and would try to take over anything they are implanted into… It is necessary to use other cells as well in order to make the armor stable."
But the question is, "Whose cells should I use?"
'Should I use a White Zetsu?' initially Kuroto has such an idea, however, upon thinking it over, he shook his head, because even white Zetsu is not invulnerable to Hashirama Cells. If a lot of Chakra is supplied to it, it turns into a tree and dies. Kuroto can't allow such a thing in the armor as it would put Karin's life in danger.
Therefore, he has to use someone else's cells, again the question is, "Whose cells should I use?"
Kuroto thought of all the strong people still alive in the Shinobi World, and some do seem to meet the requirements.
First is Uchiha Shisui, he has gone through Hashirama Cell implantation and survived, so theoretically speaking, his cells should be able to manage.
The second person is Nagato, he has also implanted Hashirama Cells in his body, so his cells should also be suitable.
And the last person is Obito.
He would need to test using the cells of all three in order to find out which cells have the highest compatibility. But the problem is, Kuroto doesn't have Nagato and Obito's cells with him, and undoubtedly, obtaining their cells is next to impossible at this moment.
So, the only option left is Shisui, and coincidently, Kuroto has Shisui's cells with him, "Let's first try using Shisui's cells."
With this in mind, Kuroto took out Shisui's cells from the cold storage, and after performing a few tests on them, he checked the compatibility.
"13 per cent, huh?" muttered Kuroto, looking at the readings.
Although the score isn't good enough, his cells seem to have some compatibility, but he is not sure whether such low compatibility would work for the armor.
"Do I have any other option?" he muttered with a thoughtful expression.
Suddenly, an idea flashed into his mind, "Why don't I try using my cells?"
Because of continuous exposure to Tenseigan Chakra and Ryuumyaku, Kuroto's body is no longer what it used to be. Kuroto has confidence that his cellular activity is higher compared to Shisui.
Therefore, using his cells is also an option.
'And if it doesn't work… I can use Tenseigan Chakra until it works!' With such an idea in his mind, Kuroto took some of his cells, and after performing some tests, he checked the compatibility of his cells with Hashirama Cells.
"36 per cent!" He was taken aback by the readings. This is a very good score, what's more, surprising is that this score is coming without any external input from Tenseigan Chakra.
Which means that his cells can be used!
With this clear, Kuroto immediately devised the plan and started the development of armor.
Kuroto decided to go with the ratio of seventy to thirty for his cells and Hashirama's Cells. This ratio will ensure the stability of the armor, and it will also make sure that it will be able to supply enough Yang Chakra.
And since this is the first iteration, Kuroto named the armor 'Type I Biological Armor'
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