Kenshin catching both his crew-mate and boat reached the shore of the sea(inside the bubble that is Fishman island). When he reached there, he saw Hachi selling Takoyaki.
"Hey Hachi, please help them out", said Kenshin pointing at his crewmates. Hachi looked at Kenshin because he has never interacted with Kenshin ever in his life.
"How do you know me", asked Hachi.
"I know you from Shakky and Rayleigh-san. That's not the main point here, our coating burst when we were trying to enter the Fishman island. So they are unconscious", said Kenshin.
"Okay okay, I will call Cami now as My house is far and is in Fishman district where the environment is not good", said Hachi. After that, he called Cami, who came and took the straw hats to her house. Of course Kenshin also helped her in picking the straw hats and putting them in beds.
After that Kenshin didn't wait for them to wake up and went to visit the districts. There he went to a clothing shop to pick up some clothes for the journey. Then he went to various cafes and restaurants to eat.
One of the things that Kenshin likes about One Piece world is the taste of food and Fishman island as expected didn't disappoint him. Kenshin was enjoying good food.
After traveling around for a bit, Kenshin decided to check with them for a bit. When he reached there he found that Zoro and Franky are not here. He thought, "Franky would be fine as he will just go to Thousand Sunny and talk with shipwrecking from Tom's brother. But Zoro is the problem. It's not like his strength is a problem but his navigation skills are way poor".
It was at this time that Robin woke up. She looked at Kenshin and said, "Thank you, you saved all of us".
"No need to mention, we are crew-mates and I just did my job. And who wouldn't want to save such a beautiful lady", said Kenshin. After that Kenshin realized what he said but he let Nature take his course.
"Ara, Kenshin flirting with a lady older than himself",said Robin teasingly. Kenshin was embarrassed a bit and thought he is way older than her.
"Okay I was just teasing. Let's go to check out Fishman Island", said Robin after he saw embarrassed. She wanted to tease a little bit but she didn't.
"Okay, let's go. I have visited many placed, so I know where the best clothing and foods are", said Kenshin.
After that, they visited cafes, malls, amusement parks and many other famous placed of Fishman island. Kenshin felt nice to have someone on his sides. He was at first feeling guilty thinking about Shigure but with his intellect he know what Shigure would want for him.
Similar to Kenshin, Robin was also happy. Although she was feeling nice with her friends in Straw Hats it still feels something different to be with a boy who understands her a bit. Robin even though mature has her childish sides which she could somehow able to show to Kenshin which surprised her also but she alike Kenshin thought 'let nature take its course'.
After that Robin and Kenshin rode on a ferry like thing which took them to place where Poleglyph was. In the ferry, they talked about many things, about each other's preference, what they like and whatnot. They talk about all this stuff.
They didn't talk about each others past as they were not that close but it was a good thing for the start. And it took them 20 minutes by ferry and another 20 minutes of walking to reach the are where poneglyph was.
The ocean forest look really beautiful like heaven on earth. Kenshin also saw poneglyph for the first time in his life. He was interested in it, so he went there and checked what material it was made up of. Kenshin reached up to the poneglyph and used his full power Haki, martial arts mastery of master level along with his full physical power to punch the poneglyph.
The area of impact was destroyed. The shock wave spread really to a large distance making Robin who was about 100 meters away stumble back another 10 or so meters. Kenshin looked and saw that the poneglyph was still intact like his punch was nothing.
"What are you doing", said Robin while checking the poneglyph is all right or not. She was really surprised by Kenshin's power and even more so by the poneglyph's indestructibility which perked her interest even more.
"No, nothing. I was just checking if it really is indestructible as the rumors says", said Kenshin. After that Kenshin went to the sides and Robin started reading it.
Kenshin saw Robin reading this intently so he was a bit interested in it even though he knows about it. He asked, "What is written there".
"It is like an apology letter written 800 years ago. A pirate named Joyboy is asking forgiveness to the descendant of Fishman island as he would not be able to fulfill his promise of taking them above the sea", said Robin.
Hearing this Kenshin was interested in finding the truth of the hidden century. Kenshin said, "I am now really interested in what happened in the hidden century and what caused the World Government to go so far as to punish whoever tries to uncover the secrets".
Like this Robin and Kenshin talked with each other about it. It was like a switch, when Robin found that Kenshin is interested in Void century she began to talk with him more intently about it. She was more open about herself than before.
(It is similar to Boa Hancock when Luffy didn't judge based on his slave mark and her devil fruit powers didn't affect her. She started to like Luffy.)
As Robin and Kenshin were happily chatting with each other, the Fishman island was on the verge of destruction. Even though Caribou didn't abduct mermaids and the royal guards didn't try to capture them because of that.
Everything else happened exactly the same due to Madam Sherly's bullshit future prediction. So, starting from guards capturing them to Hody Jones appearance everything else happened the same as in the anime.
Robin and Kenshin were walking towards the famous place where Queen Otohime was buried. But when they reached there, they saw Thousand Sunny was near and Jimbei and Luffy were arguing about something. Then Jimbei punched Luffy with his Fishman Karate.
Robin was about to go and stop when Kenshin stopped her and dashed towards them. It was then Luffy also moved to attack and Jimbei also punch. Kenshin just got in between them and used both his hands to catch them.
They were all surprised as Kenshin was able to catch them effortlessly. Only Sanji was not sure as Kenshin blocked his full power strike.
"Cool down you guys. Gentlemen talk not fight. Okay", said Kenshin and threw them both a few meters back.
"Robin and Kenshin, what are you doing in the forest", asked Chopper.
"It's a secret", said Robin teasingly. Sanji who saw this become enraged and shouted, "Oye you pretty boy. What did you do to my Robin-chan". Kenshin just ignored him like an idiot.
"Blood Devil Kenshin what are you doing with Straw Hat Pirates", asked Jimbei.
"Ohh I joined them. And are you or are you not going to join" asked Kenshin.
"I will join them when the matters will be settled", said Jimbei.
After that, they discussed what they should do and how they should save Zoro and the others and defeat Hody Jones in a way to not let Fishman Hate humans. After that, they settled on a plan like in the anime. Luffy will be inside the mouth of Shirahoshi's pet and make a surprise attack on Hody Jones when Hody Jones revealed the truth about him killing Queen Otohime that has triggered more hatred between the Fishman and humans.
After that as planned Shirahoshi and Jimbei moved towards the plaza with Luffy inside Shirashoshi's pet. In the meantime, Kenshin and the gang were using and moved towards Ryugu's palace. Just as they reached near the palace, Zoro and others have saved themselves so they didn't waste their time and moved towards the plaza.
While they were moving Kenshin said, "It will be a perfect test for you guys to test your powers. You all had improved but in the fight, many of you can still improve like Nami, Robin, Chopper, Usopp, Brook, and Franky who have just awakened Haki. Finish the fight quickly and don't underestimate enemies like Luffy. Go all out from the beginning".
Like that they reached the plaza when they reached the plaza Luffy has just punched Hody Jones and Nami used her illusion and stole the letter to world government by Queen Otohime.
After that, the fight happened like in the anime. Kenshin didn't participate in the fight and just protected them from the sidelines by using his sword domain. The fights were very easy for straw hats who all had learned Haki and improved their physique a bit compared to the anime counterpart.
Kenshin also got a surprise from the battle. He was just protecting everyone using the sword domain but one time Robin was attacked from the behind and she couldn't dodge it in time and Kenshin's domain was still a few meters away from Robin.
If the attack hit Robin, she would be at best seriously injured. So Kenshin couldn't let that happened. Kenshin looked at her and shouted "Robinnnnn". She looked at the man whom she had a crush on even though him being more than 10 years younger than her. She just smiled at him.
And in the moment of desperation, his sword mastery which has stagnated for about 7months in Low Legendary level improved to mid Legendary level extending his sword domain from 50 meters to 150 meters of radius around him.
It was then the attacker along with his sword were cut to pieces by Kenshin's attack. After that Kenshin went to check that she was okay.
"Are you okay", said Kenshin while looking at her with concern.
"Ara concern about me? Are you starting to like me?", said robin in her usual teasing tone.
"Maybe", said Kenshin while thinking. It was then Robin who was embarrassed and moved away and started to fight with other enemies to vent out her frustration.
Kenshin just looked at her and thought, "Holy shit, does that mean Robin has a crush on me. Wow hehe". Kenshin just stood there and sheepishly laughed. After that, he continued helping his friends.
The fight ended up pretty quickly with Kenshin telling them to be serious from the start and Luffy also being stronger than before defeated Hody Jones but still, hell-bent on destroying Noah. Seeing this Kenshin dashed towards Luffy to stop it as it is one of the pieces to solve the puzzle of Void Century.
Kenshin went behind Luffy who was like a mad bull destroying the ship and hit behind his neck with a strike to make him unconscious. The massive sea king has come to stop Luffy and talk with Shirahoshi(she is Poseidon and has the power to order every sea king. Momonosuke has similar power to order Zou to fight)
Kenshin looked at the sea kings who were talking about Shirahoshi being their king. Kenshin looked at them and said, "What are the story with you guys and Joyboy pirates".
The sea-kings who heard this were shocked and they spoke: "Do you also have the voice of all things like the man and another man from years ago".
"You can just say my Haki is very strong and it is something I got along the way. Enough about that and tell me about Joyboy and what happened to him", said Kenshin.
"Joyboy was one of the heirs to the Ancient Kingdom that has fled when their kingdom was destroyed by the other 20 kingdoms who feared the power of the Ancient Kingdom. When he got enough power he challenged the current rulers but didn't return. I guess he didn't succeed. He promised the Fishman to bring them above and gave them equal rights as humans", said the sea kings.
"What else do you know about the Ancient Kingdom", asked Kenshin.
"That you will have to find young man", saying that they went away pulling Noah with them.
After that, they captured the New Fish-man pirates and Luffy taking Jimbei's blood and many other things happened. After that Kenshin returned with his crew-mate to the Ryugu palace where they hold a banquet to celebrate their victory.
Kenshin saw Robin going to Neptune to ask about "Joy Boy". There Neptune told her about everything he knows about "Joyboy", "Noah" and the promise that their ancestor had made with "Joyboy". Kenshin came and told about his findings to Robin and Neptune.
"Well let's not say these to anyone as it will harm Princess Shirahoshi and we will see what happens in the future", said Kenshin.
"Yes and now let's go to the banquet", said Neptune. Like this, they went to the banquet where eating competition between Kenshin and Luffy started. Luffy with his rubber ability could eat more but Kenshin is not to be underestimated.
They both finished about half of the royal palace stalk. In the end, it was Kenshin's victory and Luffy just fell asleep after eating too much. This again shocked them as Luffy was like an inflated balloon about to burst but Kenshin belly didn't increase much.
"How can you eat so much and your body didn't even thicken a bit", said Usopp and Chopper.
"That's my secret", said Kenshin.
"Oye you bastard if you eat this much we will die out of starvation", said Sanji. Nami and others nodded.
"Don't worry, I can eat very much but can still be fine even when I didn't eat for days. This is how I survived in New World while traveling alone", said Kenshin.
"How much can you eat at a time", said Chopper.
"Ohh I don't know. One time I was hungry and I ate a horde of sea kings", said Kenshin with a smile. But those who heard his words were shocked beyond belief. "A man-eating a whole horde of monsters. His name is well suited" They all thought the same.
After that Kenshin along with the straw hats went to the treasury to keep them as it was instructed by Neptune as a gift. While returning to the ship with a handfull of treasures, they saw Peckoms and Tamago shouting something.
Kenshin let everything happened as usual. Luffy claiming Fishman pirates as his and picking up a fight against Big Mom. Tamago and Peckoms rushed to kick beat Luffy but they have just moved when the ground just ahead of them was cut and a voice was heard, "Move any further and you can say goodbye to the land of the living".
Peckoms and Tamago then saw the man who spoke. They were shocked to see Kenshin.
"Blood Devil Kenshin, the uncrowned 4th emperor after the death of Whitebeard, what are you doing here", said Tamago in a panicked tone. Yes, many have spoken Kenshin as an unspoken emperor as his strength is way above the Yonko commanders and just below the Yonko's.
"Ohh I have joined that guy's crew for fun. Tell your Mama to stay her hands away from Fishman island or I will have to go to Toto land to pay a visit", said Kenshin.
"Hmph. I will relay your words but don't get cocky. If you come to Toto land it will be your funeral there", said Tamago while retreating.
"Wow Kenshin you really are famous", said Luffy.
"Yeah just famous but enough to create fear in the hearts of Yonko's", said Kenshin plainly.
After that, they went to the ship to load all the treasures and necessary things to go on the journey. After that, the straw hats said goodbye to the people of the fish-man island and went to the New World.