One Piece: I will become stronger by checking in

One day later~

In a certain part of the East Blue, aboard the Going Merry.

"Hey, Luffy, what kind of bird are you drawing?" Kai's eyebrow twitched as he looked at the pirate flag that Luffy had scribbled in front of him.

"Are you sure this is supposed to be a skull?"

"Usopp, you take over. This blockhead of a captain can't handle this!" Kai groaned.

"Hah, I knew you'd leave the important stuff to me!" Usopp boasted, puffing out his chest. "No problem, I got this covered!"

After a short while, everyone gathered to admire the pirate flag Usopp had drawn, all smiling in satisfaction.

"Oh yeah, this is way better than Luffy's," Zoro remarked.

"For sure! Hahaha…"

Suddenly, a strong gust of wind blew across the deck, making everyone turn towards the horizon.

In the distance, an island shrouded by cumulus clouds appeared.

"Is that…?" Nami gasped, her eyes widening in disbelief. "There's no mistaking it—that's Treasure Island, the legendary island rumored to be filled with treasure!"

"(☆▽☆) T-Treasure?!" Luffy shouted in excitement, stretching his neck towards the island. "Nami, let's go! I can't wait to get my hands on that treasure!"

The Going Merry steadily approached the island's coast, and after anchoring, the crew disembarked.

Meanwhile, Kai heard a familiar and pleasant sound in his mind.

Ding! You have successfully checked in at Treasure Island! Your Indestructible Vajra Body has upgraded to Intermediate Level!

As the notification faded, Kai felt a surge of overwhelming power welling up from within. The force was so intense that it nearly made him lose his balance.

Seeing Kai stumble, Zoro couldn't help but raise an eyebrow. "Hey, Kai, what's wrong with you? Hungover from last night? You lightweight."

"Tch, when it comes to drinking, even ten of you couldn't keep up with me," Kai retorted. But in truth, he needed to sit and adjust to the flood of energy coursing through his body. "You guys go on ahead. I'll stay and watch the ship."

Without another word, Kai vanished and reappeared aboard the ship, heading straight into the cabin.

"What's with him?" Zoro muttered, still suspicious.

"Well, since Kai's staying, let's go find the treasure ourselves!" Luffy declared, sprinting toward the island. "Treasure, here I come!"

As the rest of the crew ventured onto the island, Kai sat on the Going Merry, allowing his body to adjust to the surge of power. His strength had at least tripled since his breakthrough.

While the number might not seem staggering, considering his original strength, the boost was far from simple. The new power was immense—enough to handle ten of his previous selves with ease.

He grinned, feeling more confident than ever. His Indestructible Vajra Body was truly incredible. If this level of improvement continued, he might eventually surpass even Big Mom's Iron Balloon body.

This is a game-changer.

Kai was itching to test his new strength, especially since he'd been in the One Piece world for a while now but hadn't fully unleashed his full potential. His estimations were based on his knowledge from the original story, but now he had a chance to confirm it.

A smile tugged at the corner of his lips. I can't wait.

After spending the day alone on the ship, Kai finally heard the familiar sounds of Luffy and the others returning. He strolled out onto the deck.

"Hey, Kai! You really missed out today! We saw all kinds of crazy stuff—like this old guy growing out of a box!" Luffy babbled excitedly.

Kai listened, occasionally chiming in with a comment or two. The Going Merry was soon filled with laughter once again.

The next morning, Kai was still sleeping in the cabin when the sound of commotion from outside woke him up. Rubbing his eyes, he stepped onto the deck.

As soon as he emerged, he saw Luffy and Usopp bowing deeply to someone.

"Johnny?" Kai asked, recognizing the familiar face of Zoro's old companion. "What's going on?"

Kai walked down to the others and asked Zoro for an explanation. After a quick rundown, he pieced the situation together—Johnny believed that Luffy and Usopp had killed his partner, Yosaku.

Kai calmly approached Yosaku, remembering the events from the story. He turned to the others and said, "Hey, if you want to save him, grab some fresh oranges from Nami's stash, or he really will die."

Johnny, eyes wide with hope, asked, "W-What?! You mean Yosaku's not dead?"

"Not yet, but give it some time if you don't act fast," Kai said with a smirk, his tone casual but firm.

Luffy and Usopp immediately bolted to the cabin to retrieve the oranges. Finally, they're being useful, Kai thought.

Once Yosaku was fed the oranges, his complexion gradually improved.

Johnny, seeing this, was overcome with emotion. He rushed over and hugged Kai's leg.

"Brother, I'll never forget your kindness! You've saved my—"

"Yeah, yeah, get off me," Kai muttered, shaking him off.

Later, Kai sat on the ship's deck, giving the crew a quick lecture on the importance of a balanced diet at sea.

"So yeah, eating right is important. Otherwise, you'll end up with problems like this," Kai said, finishing his lesson.

Luffy, suddenly standing up, shouted, "Then let's go find a chef!"

Everyone nodded in agreement. This time, no one argued with Luffy's idea.

Johnny, hearing this, chimed in, "If it's a sea chef you're after, I know just the place!"

"Really? Where?" Luffy asked, eager for the next adventure.

Johnny narrowed his eyes and, with a dramatic pause, said, "The Baratie—the Sea Restaurant!"