One Piece: I will become stronger by checking in

After hearing this, Luffy's expression was blank at first, but then his face lit up with excitement.

"Really... really? You're really joining our crew?"

"Yeah, it's true!" Kai confirmed.

"Ahahaha, that's awesome!" Luffy shouted, filled with joy.

As Sanji, Luffy, and the others made their way to the ship, a voice called out from behind just as they were about to board.

"Don't catch a cold!"

Sanji's eyes instantly filled with tears, and they flowed uncontrollably down his face. He turned around to face Zeff on the second floor and dropped to his knees.

"I will never forget your kindness, old man!" he cried out.

Zeff's eyes, too, glistened with emotion. "Go! Chase your dream…"

...

Some time later, on the sea heading to Cocoyasi Village, the crew enjoyed a leisurely meal aboard their small boat.

Kai took a bite of meat and followed it up with a swig of wine, sighing contentedly. "Now this is living!"

Before Sanji had joined, meals had been rough. Even though Kai could summon plenty of meat from the System Space, it got tiring after a while. Sanji's cooking was a luxury.

"Yeah, but I'd rather be cooking for Miss Nami~" Sanji said dreamily, thinking about their next destination. "I can't wait to see her again!"

Suddenly, the ship started to shake.

"Huh? What's going on?" Luffy asked, puzzled.

In the next moment, a massive creature burst out of the sea, towering over the small boat.

"This... this is huge! What's a beast like this doing in the East Blue?!" Usopp shouted in fear, clutching his head. "Aren't these creatures only found in the Grand Line?"

"Oh, wow, a giant sea cow!" Kai commented, raising an eyebrow.

"Idiot! That's not a sea cow!" Sanji corrected. "It's a sea hippo!"

"Does your hippo have horns?" Kai smirked.

"Well… if you put it that way…"

"Stop arguing! It's about to attack us!" Usopp screamed.

"Alright, alright! I was just thinking the boat was a little slow. Guess we've got our solution now," Kai said calmly. With a flick of his hand, he unleashed his Conqueror's Haki toward the manatee.

Instantly, the creature froze in place, staring at Kai with pure terror. It didn't dare make a single move.

"Well, that makes things easier," Kai grinned.

"Brave Niu Niu, charge!" Luffy and Kai stood at the bow of the boat, shouting as they tied the creature up with a rope.

"Oh-ho-ho! We're moving so fast!" Luffy cheered.

"Full speed ahead to Arlong Park!"

As the island came into view, it was clear they had reached Cocoyasi Village, where Arlong's base was located. But, as usual, things didn't go as planned.

"Hey, hey, dumb sea cow! Slow down!" Sanji yelled as they approached a wall at alarming speed.

But Kai and Luffy were too caught up in the excitement, shouting, "Go, go, charge!"

Before anyone could react, the manatee slammed into the wall, launching the boat and its passengers into the air.

"Why does it feel like we're flying?" Luffy wondered aloud.

Ding! A system notification rang in Kai's mind:

[Check-in at Cocoyasi Village successful. Fruit ability upgraded to intermediate level. Piercing Eyes unlocked!]

Suddenly, a sharp pain shot through Kai's eyes. He clutched his face, sitting down as the sensation overwhelmed him.

"Kai, what's wrong?" Luffy asked, noticing Kai's strange behavior.

"It's nothing, just a little pain in my eyes," Kai replied, trying to shrug it off.

When the boat finally landed, the stinging sensation in Kai's eyes began to subside. He stood up, feeling something had changed. As he opened his eyes, anyone who looked at him would've noticed his pupils had turned golden, glowing with brilliance. His appearance had become even more striking.

But that wasn't the only change. Kai found he could see through his crewmates—not their clothes, but their internal energy. In his eyes, they appeared as beings made of Qi and blood, with their circulatory systems glowing brightly.

By observing the flow of their energy, he could now detect weak points and injuries with perfect precision—like a vulnerability detector. On top of that, he realized his eyes could release powerful energy beams, similar to Superman's heat vision from the stories he'd heard.

This is amazing! Kai thought, thrilled with his new ability. He was growing stronger by the day.

Just as Kai was reveling in his new power, another system notification rang in his mind.

[Ding! Check-in at Cocoyasi Village successful. Demonized form of the Monkey King Fruit unlocked!]

[Error detected. System bug unable to repair. Warning: Demonized form is extremely unstable. Host needs to...]

Kai's excitement turned to confusion. The system was malfunctioning, yet it had rewarded him with something new. A demonized form? It sounded powerful, but unstable.

Before he could grasp what was happening, an overwhelming wave of killing intent filled his mind. Instantly, Kai collapsed onto the deck, rolling around in agony, clutching his head.

The others, alarmed by the noise, rushed over. When they saw Kai writhing on the ground, they froze in shock.

Luffy quickly knelt beside him. "Kai, what's happening to you?"

The rest of the crew gathered, concern and fear written all over their faces.

"What... what's going on?" Sanji stammered, unable to hide his fear.

Zoro said nothing but his expression mirrored Sanji's worry. They had seen how powerful Kai was during his battle with Hawkeye, knowing full well he was leagues stronger than any of them. And now, that same man was rolling on the ground in pain.

What on earth could be happening to him?