I'm writing this on a whim after reading Ten Year Difference by Stormy1x2

This is also an age swap fic. Here, Luffy is the older brother instead of Ace. I'm still in denial over Ace's death. Comments are welcomed and will be cherished!

The Whitebeard Pirates laughed as they watched their newest addition get thrown overboard again, Namur diving in after him, after his ninety-fifth assassination attempt on their captain. The kid was an absolute riot. First the rookie had challenged Oyaji, then he defeated Jinbe and now he was trying to assassinate their captain. The crew hadn't had this much entertainment since Strawhat Luffy had set out to sea years ago, declaring wars left and right, bringing down warlords and then becoming a Yonko after defeating Kaido.

Apparently he didn't want to fight Whitebeard because "his mustache is cool" and that he seemed like a "cool giant-ossan". With Big Mom, he had somehow bonded with her over their mutal love for food. From what the Whitebeard Pirates had heard, Luffy had somehow managed to change Big Mom's mindset and she had stopped terrorizing her people, though she was still terrifying to her enemies. There was some unknown relationship between Luffy and Shanks so they didn't fight but apparently Strawhat didn't like Kaido and some things he had done to the people of Wano. So the pirate had proceeded to challenge Kaido, beat him up, and take over his title of Yonko. Now their Oyaji and Strawhat had an established relationship where they meet up once every two months and the two crews would party together. Lots of booze and high quality food? No one on the crew complained.

Speaking of meetings, today was the day where the Strawhat Pirates are coming to meet and party. They had a good relationship and no one wanted it to break down because of one challenge-happy fire logia user. Some of the commanders were hoping that Ace won't just launch himself at the Yonko, trying to kill him and would then most likely be killed by the Strawhat's crewmates. So after a quick game of rock-paper-scissors, the looser, Thatch walked over to Ace to explain to him today's situation. The fire user was leaning against one of the many barrels on deck, dripping wet. When he saw the cook walking over, he had a guarded expression on his face. Ignoring the kid's apprehensive behaviour, Thatch unceremoniously plonked himself down next to the rookie. "Look kid, we are going to have a visiting crew today so whatever you do, don't try to challenge them alright?" Hearing that, Ace lifted one eyebrow. "Why not? And since when did pirate crews just visit each other?" Hearing that, the commander just sighed, "It's a…weird relationship. We just…somehow get along. Anyway, whatever you do, please don't try to kill any of them." then stood up and left.

Later that afternoon, when Ace was napping in his room, the Strawhat Pirates arrived. "Yo, Old Man! We brought lots of food and alcohol!" The smiling captain shouted the moment he came onboard the Moby Dick, his crew right behind him. Since he had started the pirate life five years ago, Strawhat Luffy had achieved an impressive bounty of two and a half billion beris, his first-mate not far behind with two billion. Apparently the pirate hunter's bounty had drastically increased after he had lopped of Admiral Akainu's head with a single swing of his sword. The rest of the crew had decently high bounties for New World pirates.

The two crews settled and were now trading stories and updating each other on what was happening in their territories. It was more of a gossip and complain to each other session really. "Torao is being mean again. He didn't want to meet up and eat with me yesterday.", "Don't worry Luffy, I heard that he's in a prissy mood because some rookie crew tried to attack him while he was on vacation in the East Blue." could be heard. Apparently the Surgeon of Death, having become a Shichibukai two years ago, was like Mihawk and Shanks now. The doctor had decided to travel outside the Grandline aimlessly, saying that he needed some time off to regain whatever sanity that he had lost while being in the company of Strawhat.

Just as the respective cooks of the two crews started to grill food, a drowsy Ace walked onto the deck. "I smell food, what's going on?" He was still rubbing his eyes and had yet to notice who the other visiting crew was. Before he could open his eyes, a loud "ACEEEEEEE!" was heard and the next thing he knew, he was getting thrown across the deck with someone hugging him around the waist. Both crews could only watch in horror as they saw the Yonko throw himself at their newest (still trying to convince him) addition, resulting in both of them tumbling across the deck. The Whitebeard Pirates had hoped that Ace would not throw himself at the Yonko. They weren't expecting the Yonko to throw himself at Ace though!

"What the hell Luffy! Didn't I tell you to stop throwing yourself at people?" The phrase that he used to tell his older brother everyday came out before he could even register the situation properly. "Wait, what? Luffy? On Whitebeard's ship? What?" Then the situation started to sink in. "You're the visitor?" Ace at his point was shaking Luffy by the shoulders back and forth and Luffy just continued laughing at him, not at all concerned by the panicked look on his younger brother's face. Ace was broken out of his hysterical moment when Marco tapped him on his shoulder and asked, "How do you know Strawhat-yoi?" He wasn't the only one that was curious as to why Ace had yet to set fire to the other Yonko. It was obvious that they know each other and are close, but what was the exact nature of their relationship?

Both D's turned to look at him, pointed at each other and said, "He's my brother."

All the Whitebeard Pirates just blinked at the siblings for a few seconds before scream a loud "WHAT?" that could have probably be heard all the way to Marineford. "You have a brother?" Izo pointed at Luffy and continued in a disbelieving tone, "He's your brother?" Ace just looked at him with a 'duh' look.

Marco looked on and then realized, "Oh shit, we kidnapped the younger brother of a Yonko! Sure, the Yonko is Strawhat, probable the most cheerful Yonko, but it's still a Yonko!" By this point he could feel a headache building up.

Wait, what is Ace doing on this ship?" All the pirates froze when they heard this question and wondered who will be brave enough to answer this question. Thankfully, Oyaji took it upon himself to answer, sparing his children the meltdown. "Gurarararara! I want him to be my son!" Strawhat stilled when he heard that. "Oh? You want Ace to be your son?" As soon as he said that, there was a heavy blanket of pressure that fell on everyone.

Conqueror's Haki.

Strawhat Luffy's haki was known as one of the most powerful haki out there, capable of bringing down whole structures with it. But this haki was different, it wasn't aimed at knocking out anyone. It was more of a warning, that if the reasoning behind taking Ace didn't suffice, then the full pressure will come down onto them. The rubber captain was standing in the middle of the deck before Whitebeard, the infamous strawhat shadowing his eyes.

Whitebeard looked at him before answering the unspoken "Why?" that Luffy had asked. "He has a good look in his eye. Besides, I do not care where someone came from, everyone is a child of the sea." Whitebeard grumbled out. Oh yes, he knew that Ace was the son of his old rival. How could he have not? The child was a perfect mix of both his parents.

The pressure held on for a few more seconds before it was let up. Strawhat let out a wide and cheerful smile, "Shishishishi. Then I'll let Ace stay here then." and all the onlookers let out a breath that they did not know they were holding.

"Wait, what? Luffy! What do you mean?" It seemed that only the person of the entire conversation was still against this. Ace did not know what was going on. Also, what was that pressure from before? The only other time he had seen his older brother like that was before Ace had set out to sea. That time, Luffy had come back to Dawn Island for a visit and saw a bunch of rookie pirates threatening the village people near the docks. Ace had several bruises on his face from protecting Makino and the Mayor as they had been standing at the front of the crowd. Ace was strong, but even he couldn't fight against an entire pirate crew while still trying to protect innocent bystanders. Luffy and his first-mate had arrived at the docks right as the captain of the rookie crew had drawn a sword and was about the bring it down on Ace.

Next thing Ace knew was the captain falling down into a heap and foaming at the mouth. The crew parted like the red sea to see who had arrived. Standing there with his arms crossed was the calmest Ace had ever seen his older brother be. Luffy walked towards Ace, walking pass the frozen pirates, before bending down to help him up. Only when Ace got to his feet did the pirate crew creak themselves out of their shock and yelled at his older brother. "Who do you think you are? We are the Trench Pirates and we have a total bounty of a hundred million beris! So get out of our way before we hurt you!" Luffy turned around to face them, his face showing no emotions or whatsoever. Then he tilted his head slightly and spoke to his first-mate,"Zoro, take care of the trash. I'll be in the bar." before turning around and guiding his brother and the villagers away from the pirates.

"You didn't even have to say captain." Zoro could be heard saying before drawing out Wado Ichimonji and proceeded to cut down all of the pirates standing there. The villagers wisely ignored the screams that erupted from the pirates as they walked away. That day was one of the rare times where Luffy had behaved like that. Gone was the usual cheerful and happy-go-luck attitude. In place was the disposition of an Emperor.

Now Ace had seen that Luffy again and was in awe, but it did not distract him from the fact that his brother had just given Whitebeard the 'go' sign to keep him here on the ship! Right when he was about to scream and shout at his older brother, Luffy drew Ace into a tight hug, effectively preventing him from throwing an tantrums. Just as he was about to open his mouth and scream into Luffy's ears, his brother spoke up in a quite voice. "Trust my decision as you had always done so in the past. I am your older brother Ace. Everything I do for you will always be from the bottom of my heart."

Ace stilled in his brother's arms as soon as he had heard his brother speak in that tone. Luffy used to speak to him in that voice when they were younger, when they were still together with Sabo. Sometime when the two younger boys had nightmare, Luffy would talk to them in that soft soothing voice, telling them stories of kingdom and adventures, distracting them away from the nightmares that plagued their dreams. So, Ace listened to his brother. "I won't stop trying to kill the old man brother. But I'll try to behave a bit better with the crew." Ace muttered into his brothers shoulder.

"That's all I'm asking."

After that last sentence, Strawhat broke away from his brother and demanded his cook meat, acting as if nothing had happened. So the rest followed suit and the two crews drank and ate well into the night.

Two months later when the Strawhats came to visit again, it was to the sight of Ace welcoming them onto the deck with a huge tattoo of Whitebeard's jolly roger on his chest. Luffy took one look at it and asked Ace, "Are you happy?" A soft smile ghosted across the fire logia's face before he answered his brother, "Yeah, I am." looking directly in his eyes. "Shishishi, that's all that I need." Luffy answered before grabbing his brother's arm and dragging him over to where the cooks were having a competition.

Ace looked at his brother's back while being dragged toward the crowd. He would not trade Luffy for the world.

A/N: In the near future, I will be posting a Harry Potter X One Piece fanfic. I still haven't decided on the title and will edit this a/n once I do think of one. It will be posted on both FFN and AO3. Hope y'all look forward to it!