One Piece is a pirate adventure that is nothing without its happy-go-lucky captain, Monkey D. Luffy. Entranced at a young age by the magic and romanticism of pirate adventures, his dream is to become the King of the Pirates. That dream is the core of the One Piece story, as his growth to become strong and journey to find the coveted One Piece have taken him and his Straw Hat crew on a grand adventure full of life changing choices and discoveries.
And while it may not be obvious to the outside viewer, there is consequential and consistent change from all of this, and it is most apparent in the main character himself. This list will be looking at those notches in the door frame, as it runs down a few ways that Monkey D. Luffy has changed since episode 1.
10 Crew Size
The most obvious marker for Luffy's progress across the Grand Line is his crew size. As he's progressed across every island and inch of sea, he's accrued a larger and larger crew. East Blue brought fans his first mate, navigator, sniper, and cook. The first half of the Grand Line, Paradise, brought him his doctor, archaeologist, shipwright, and musician.
And, in the New World, he gained an entire armada in the form of the Straw Hat Grand Fleet. He wouldn't be a captain without a crew nor a king without a massive kingdom.
9 Bounty Size
While crew size may give fans a Pokemon-esque sense of progress and change, the more technical based anime fan may appreciate hard numbers a little more. And, in One Piece, there are no numbers bigger and more shifting than bounties. They're like power levels except their size represents more story potential than any actual danger (though that shouldn't stop bounty hunters from being scared).
In Luffy's case, each bounty increase he's received is a testament to his various exploits across the sea, such as the defeat of Arlong, the destruction of Ennies Lobby, and that one time he freed a bunch of dangerous criminals from the anime Alcatraz. His bounty as of this writing is 1.5 billion berries which is a stark increase from his first bounty of 30 million and the bounty he had on episode one: 0.
8 Enemies
Bounties are perhaps the most objective and least emotional way to look at a pirate's infamy. If one really wants to see how dangerous a pirate really is, they should take a good, hard look at the people that want them dead. In Luffy's case, there are quite a few. The Marines alone want him on a guillotine, with Marine standouts like Smoker, Rob Lucci, and Akainu each wanting a piece of him for personal reasons.
In the pirating world, Luffy is wanted by 3 out of 4 of the Emperors of the Sea. He's also defeated three Warlords and has stolen a lot of people's food. In this world, being a good pirate means being a hated pirate.
7 Leadership
Something that may be obvious to outside viewers and is glaringly obvious for long time fans is that Luffy is not a conventional leader. His leadership skills haven't really been tested that often in the series, and he's actually jeopardized that quality in more than one occasion (looking at you Davy Back Fights). However, that doesn't mean that he hasn't learned a thing or two about responsibility.
On Ennies Lobby, Usopp dishearteningly left the crew over a dispute only to want to come back after the events of Water Seven. However, the question on everyone's mind (or at least Zoro's) is how easy should it be for a crew member to return after quitting. This arc really tested how respectful the crew had to be of their captain and how headstrong Luffy needs to be to be a faithful leader. With Zoro's surprising help, the Straw Hats weathered through, and Luffy came out looking like a stronger captain than ever.
6 Wisdom
If there's one thing that Luffy isn't, it's wise. A lot of his sustainability before the time-skip is chocked up to pure ambition, luck, and plot armor. And while much of that hasn't gone away in the New World, the Paramount War did encourage him to start thinking critically about how he handles every situation.
His training with Silvers Rayleigh would not just help him master Haki, but the Dark King would also help him practice being more tactful in the battlefield. This has come to mean making more purposeful decisions in combat and being more tactful with his attacks, such as how he now chooses targets whenever he deflects attacks with Gum Gum Balloon.
5 Character Design
The character designs were already colorful enough early on in the series, but Oda did prey upon recycling certain costume designs during those youthful years. Once the series progressed to more ambitious and culturally distinct areas, he played around more with how the Straw Hats looked.
For Luffy, this meant subtle changes from his initial red shirt and jean shorts. Since entering the Grand Line, Luffy has worn robes, jackets, and different colored shirts. After the time-skip, he now has his 3D2Y tattoo, his scar from Admiral Akainu, and is constantly going open shirt for every adventure. That's not even touching the distinct costumes that he's worn during every arc, such as gang suits and kimonos.
4 Family
When the series began, fans didn't know too much about Luffy's family, and it didn't seem like the story even cared that much about it. For a while, fans were even convinced that "Red-Haired" Shanks was his dad. However, after bombshell after bombshell, fans have gotten a much better picture of what his family looks like. When the Straw Hats landed on Alabasta, they learned of Luffy's hot headed older brother.
When they were on Water Seven, they learned about his famous grandfather and infamous father. And, during Luffy's latest flashback arc, fans got to see his last adopted brother. Luffy's history is already long and complex, but fans still eagerly await for his mother's reveal.
3 Poison Resistance
As Luffy first set sail to build his crew, the one power that was fans weren't expecting him to get was poison resistance. It isn't exactly a regular system, and it seems a little specific. However, almost every fourth villain in the series nowadays has poison abilities just so they can be shocked that the poisons don't work as well on Luffy.
During the Impel Down Arc, Emporio Ivankov had to inject Luffy with special antibodies when he was poisoned by some of Magellan's strongest attacks. This in turn would give Luffy some of the strongest antibodies on the sea, and fans now have to be reminded of it every now and then.
2 Haki
Haki is the One Piece superpower that definitely did not exist when the series began. It is creator Eiichiro Oda's last minute way to explain a few cooky events in the series, like Shanks scaring away a Sea King or mantra, and a goal to build a few training arcs with.
Luffy would show slight instances of this across the early series, such as being able to sense the real Mr. 3 on Little Garden or using Conqueror's Haki during the Marineford Saga. It wasn't until he started training with Rayleigh that he would begin training and utilizing all forms of Haki consistently.
1 Gears
One major thing that would shock new fans to One Piece is how Luffy's repertoire of skills changed from just Popeye, rubberhose tactics to the Shounen craziness of his gears. Luffy's entire fighting style since day one has changed, and a lot of that is due to him adopting Gear Second, Third, and, much later on, Fourth.
With Gear Second, his most frequently used power, he is able to move at "jet" speeds by forcing his body to pump adrenaline faster across his body. With Gear Third, he is able to increase the size and power of some of his melee attacks by blowing air into his bones. Combined with Haki, he is able to use these to punch with fire or giant, black orbs for fists to create fights that fans genuinely have never seen before.