Humans in the One Piece world have unusually strong constitutions. The energy contained within their cells is several times greater than that of humans in the ninja world.
So once Nico Robin gained the ability to extract chakra, her speed in refining it was not inferior to that of a pure-blooded Senju—if anything, she was even better.
As long as she had the qualifications to extract chakra, catching up to Tsunade was only a matter of time.
For Nico Robin, Senju blood was more or less optional.
Her base physical condition was already no weaker than that of the Senju clan.
Of course, that still couldn't compare to a cheat-tier monster like Hashirama Senju and his Sage Body.
After all, Hashirama himself was a special case.
Once the cell fusion was complete and Robin had gained the ability to refine chakra, Tsunade taught her the Senju clan's secret chakra refining technique and personally guided her step by step through the process.
Robin didn't disappoint anyone. The very first time she tried, she extracted an astonishing amount of chakra.
If an ordinary person in the ninja world extracted chakra on that scale, their cells would have been drained dry on the spot, and it would likely have caused damage to their body.
But Robin felt absolutely nothing.
That made Tsunade marvel once again at the physical quality of people in the One Piece world.
They were seriously overpowered.
Once Robin had learned the chakra extraction technique, Tsunade handed over the task of teaching her ninjutsu to her grandfather, Hashirama Senju.
As for Tsunade herself, she dragged Shizune straight into the lab and joined Tobirama and Orochimaru in trying to crack the secret of Devil Fruits.
Robin had no complaints about that.
The God of Shinobi was personally teaching her ninjutsu—what was there to complain about?
So during the day, Rowan sent Robin back to the One Piece world, where she coasted along at Crocodile's side.
At night, Rowan brought her to Orochimaru's secret base to study ninjutsu.
Her life was pretty full.
Once Robin learned the Clone Technique, she left a clone by Crocodile's side.
She herself stayed at Orochimaru's base all day, extracting chakra while learning ninjutsu from Hashirama Senju.
In just a few days, Robin had not only learned tree climbing and water walking, but the amount of chakra she possessed had already reached the level of a Konoha genin.
On top of that, she had even learned a few Water Release techniques.
And as for why she chose Water Release—
Naturally, it was a little surprise she had prepared for Crocodile.
Everyone knew that Crocodile's greatest weakness was water.
In the story, Luffy had managed to take Crocodile down precisely because of that, and Robin naturally hadn't forgotten it. So when she started learning ninjutsu, aside from the three basic techniques—the Clone Technique, Transformation Technique, and Substitution Technique—
the very first thing she focused on was Water Release.
If she ever truly fell out with Crocodile, that one trick alone would definitely give him a surprise.
If she got lucky, she might even be able to turn the tables and kill him.
After all, in Crocodile's eyes, Nico Robin was just some insignificant nobody. Other than the knowledge in her head, nothing else about her was worth taking seriously.
And when a weakling suddenly unleashes power no one expected, it becomes very easy to land a sneak attack.
Of course, that kind of chance only comes once.
If it failed, then the only option left would be a head-on clash.
Meanwhile, after staying at Orochimaru's secret base for a few days, Rowan started getting bored.
Orochimaru, Tobirama, Tsunade, Shizune, and Kabuto were all so busy researching Devil Fruits that they had lost all sense of time.
Robin was focused on growing stronger and could tolerate the solitude easily.
As for Hashirama, there was no need to even mention him. His will was like iron. This kind of ascetic lifestyle was barely even a footnote to him.
It didn't bother him at all.
Rowan, on the other hand, despite having the power to travel between worlds, was still basically an ordinary person. After living in Orochimaru's base for a few days, he started to feel like he was in prison.
He was bored out of his mind.
So during one dinner, Rowan suggested going to the One Piece world for a bit.
"Where do you want to go?" Tsunade asked curiously.
"The Baratie."
After thinking it over, Rowan said, "I want to see just how good the food there really is."
"Sure."
Tsunade agreed without hesitation. Rowan was her friend, not her prisoner. There was no way she could keep him locked up here and refuse to let him leave.
Besides, even if Rowan really wanted to go, they couldn't stop him anyway.
After a moment's thought, Tsunade added, "But your safety is important. My granduncle, Orochimaru, and I all need to stay here and work on the Devil Fruit research, so we probably can't protect you personally."
"So this time, Grandfather will go with you to the Baratie and stay by your side."
Even if Hashirama was in an Edo Tensei state and had less than half the strength he once had, it was still basically impossible for anyone below admiral level to defeat him.
With her grandfather protecting Rowan, Tsunade felt completely at ease.
Hashirama also understood how important Rowan was and patted his chest.
"Don't worry. Leave it to me."
Rowan glanced at Hashirama and nodded.
Sure, his personal bodyguard had gone from an easy-on-the-eyes beauty to a lovable idiot, but Rowan didn't mind. After all, Hashirama was ridiculously strong, and in terms of security, he actually inspired even more confidence than Tsunade.
So the next morning, Rowan set out with Hashirama.
They first went from the Naruto world into the One Piece world, then headed for Nanohana, the port city of Alabasta.
But what surprised Rowan was that Nico Robin, disguised as Tsunade, ended up joining the trip to the sea restaurant too.
The reason was simple.
She knew the way.
Neither Rowan nor Hashirama had any way to object to that.
After all, for two people who had only ever read the manga, getting from the Grand Line to the East Blue was obviously not going to be easy.
Originally, the two of them had planned to hitch a ride on a ship to the East Blue.
But now that Robin had joined them, there was no need to hitch a ride at all.
As for Crocodile, Robin left a clone by his side.
After all, Crocodile had neither chakra nor ninjutsu, so there was no way he could see through it and realize it was just a clone.
Robin had even left a Den Den Mushi with the clone so they could stay in contact and exchange information.
There was basically no risk of being exposed.
And even taking ten thousand steps back—even if it did get exposed, it didn't really matter.
Because the current Robin no longer needed Crocodile's protection. Even if the World Government really did discover her, the worst case was that she would simply go with Rowan to the Naruto world.
She refused to believe the World Government could chase her all the way into the Naruto world.
And if the World Government really were that capable, they would've taken over the heavens by now.
A day later, the three of them arrived in the port city of Nanohana aboard an amphibious transport turtle.