Summary
- Luffy's victory over Kaido is a significant achievement, establishing him as an Emperor of the Sea and one of the strongest characters in One Piece.
- Arguments that Kaido wasn't at his best due to things like lifting an island or fighting multiple opponents are invalid. Shonen traditions dictate that fighters give their best in a final battle.
- Luffy had some minor handicaps to keep him in the fight, but his perseverance earned him his win fair and square.
Luffy's win over Kaido is one of his most significant achievements since One Piece started. By taking out the Strongest Creature, he took his place as an Emperor of the Sea, earned a 3 billion Berry bounty, and established himself as one of the strongest characters in the series. Unfortunately, many consider this win invalid for one reason or another. Some say Kaido lifting the island stopped him from fighting at full potential. Others say fighting multiple opponents softened the King of Beasts up for the Straw Hat captain. The general assumption is that if these two were in a one-on-one fight with no outside help or interference, Kaido would have easily beaten Luffy.
While the outcome of this battle could have been different, saying Kaido wasn't at his best is a disservice to both fighters and a dismissal of battle shonen traditions. When a shonen protagonist enters a final battle with a villain, both will fight at peak performance unless otherwise stated; anything less would invalidate the outcome. This is why many shonen villains pull out their hidden reserves or trump card near the end of such (in Kaido's case, Kaen Daiko would represent the latter); it lets the audience know they gave everything they had when the main character beat them. Luffy's final clash with Kaido was written in a way that removed all doubt that he was the stronger of the two.
"Kaido Was Lifting an Island"
One of the most common arguments for Kaido underperforming is that he was lifting the island of Onigashima throughout his fight with Luffy. This feat is impressive even for where everyone was in the series regarding strength. Even the world's strongest creature would have needed to invest substantial amounts of his power and concentration into keeping the island afloat. If so, he couldn't have given his all in any battle after launching the island. This logic isn't airtight. It was never openly stated how much energy Kaido needed to lift the island and how much he reserved for fighting. He could have been using anything from all of his strength to none of it on the island.
There's an argument to be had for Kaido investing little to no energy into lifting the island. Plenty of Devil Fruit powers can function independently of their users' input if they aren't rendered unconscious or dead. The user's strength isn't necessarily a factor, either. By this logic, all Kaido would need is to use his clouds to lift the island out of the water and let them do the rest of the work from there. This disconnect between a Devil Fruit's powers and the user's strength can be observed in Vander Decken IX's Mark-Mark Fruit. He moved the colossal Noah ship by touching it with no signs of exhaustion on his end. For how much trouble even Luffy had destroying this ship (before it hit Fish-Man island), Decken would need to be greater than or equal to him in strength to move it, which is false.
The disconnect can also be observed through Doflamingo's Birdcage. His technique remained active and pushed back the likes of Zoro, Admiral Fujitora, Kin'emon, Kanjuro, and several others. Whether the Warlord was fighting or getting beaten by Gear 4 Luffy, the birdcage continued to shrink and tear through the streets of Dressrosa until he was rendered completely unconscious. Either he, too, was severely nerfed while battling Luffy, or Kaido's Flame Clouds work similarly independent of his condition. Therefore, Kaido should have needed similarly minimal effort to keep Onigashima moving. Then, he could focus the rest of his concentration and power on fighting Luffy and Co.
"Kaido Had to Fight, Like, 13 Other Guys"
Another argument for Kaido not fighting Luffy at his best is everyone else he had to fight. In addition to Luffy, Kaido had to fight seven of the Red Scabbards, Eustass Kid, Killer, Trafalgar Law, Roronoa Zoro, and Yamato – Momonosuke. All his accumulated damage and exhaustion could have caught up to him by the time his final round against Luffy started. In fairness, Kaido was being worn down by all the fighting. This was best illustrated by the thinning Flame Clouds he used to lift Onigashima. However, this exhaustion was never enough to affect his performance against Luffy. He could still use his best moves and techniques to their fullest potential. His stamina may have been reduced, but his output remained constant.
Furthermore, Kaido's accumulated damage didn't add up as many think it did. Kid and Momonosuke only managed to cause him pain, which isn't saying much. The cuts he received from the Scabbards were too shallow to be serious (despite his theatrics while being sliced). Law, Killer, and Zoro only managed to get in one decent shot each, and only Zoro hit hard enough to leave a scar. Even Yamato admitted she was only there to buy time until Luffy returned for another round. This is to say nothing of the King of the Beasts possibly having a minor healing factor (his cuts closed up and stopped bleeding as he fought). Luffy was the only person to strike Kaido and leave consistent, severe, lasting damage.
It's also worth noting that Luffy was the only one who could damage Kaido enough to finish him off, especially by the end of the Raid. If he hadn't gotten up after he was "killed," the Emperor likely would have finished off his friends and the rest of the remaining forces. His fellow Worst Generation members might have been able to do something if they weren't exhausted from their respective fights, but there's no guarantee. Luffy was the only one with enough strength to deliver a knockout blow and secure the Raid's success.
"Kaido Wasn't Fighting at His Best"
There's also an argument that Kaido didn't give this fight his best. Besides the lifting-the-island point, there's also the fact that he deliberately refused to dodge Luffy's Gum-Gum Bajrang Gun. If he had gotten out of the way, he could have struck Luffy with his Kaen Daiko and finished him off.
However, this would be another dismissal of the shonen battle manga conventions. Dodging or blocking an opponent's best technique doesn't prove anything. Clashes between combatants' best moves establish absolute dominance and can, therefore, be used as proof that one fighter is superior to the other. If Kaido had done either of these things, he would have indirectly admitted Luffy was strong enough to hurt and even beat him. He'd already suffered this humiliation in Episode 1025 ("You dodged it, Kaido! Must be because it hurts!") and likely didn't want to do it again at such a critical moment.
Kaido's desire for this to be a good fight can't be stressed enough. At some point, everything about how Kaido acted showed how he took Luffy more seriously as an opponent – using his hybrid form, drinking as a "power-up," getting mad when Guernica helped him win, allowing attacks to hit him, etc. He didn't want to take half-measures against Luffy because he acknowledged him as a worthy opponent.
Kaido's desire for an honest battle could further suggest how little power he needed to invest in lifting Onigashima. If the island acted as the handicap many made it out to be, he would have put it down after acknowledging Luffy. He wouldn't want anything to bring the outcome of his best match in decades into question.
Finding out who was the best between Kaido and Luffy meant as much to the King of Beasts as it was to any reader. He did everything he thought was necessary to ensure he gave this battle his all and came out the winner. Luffy did the same. By the end, both used everything up their sleeves and then some to prove who was the better fighter overall.
Did Luffy Need Any Handicaps to Beat Kaido?
There were places where Luffy needed help beating Kaido. Most notably, he lost three times on Onigashima before he got his win against the King of the Beasts. The first time he lost, his Worst Generation cohorts kept him safe until he regained consciousness. The second time he lost and was thrown off Onigashima, he needed the Heart Pirates to fish him out of the water, Caribou to feed him, Momonosuke to fly him back to the island, and Yamato to stall Kaido. In a straight, one-on-one fight, Kaido would have killed Luffy before he ever learned to infuse his attacks with Conqueror's Haki.
There's also something to be said about how these two came about as close to a one-on-one match as circumstances allowed. Luffy needed all his allies in the Ninja-Pirate-Mink-Samurai Alliance to clear the Gifters, the Tobiroppo, the All-Stars, and Big Mom to get his chance to fight Kaido. Likewise, Kaido needed the Beast Pirates to weaken the Alliance's forces so they couldn't step into the late phases of the fight to help themselves. If it weren't for this Raid, more people like Guernica would have stepped in and turned this into much less of a fair fight than it was.
Besides that, however, this battle went about as well as it could have. Luffy needed four tries but finally got one good win over the strongest creature, and it was when they were both fighting at their greatest potential. There could come a day when Kaido comes back more of a threat because he's not lifting an island. Until then, Kaido should be assumed to be at his best on Onigashima. With that, fans can say Luffy earned his win and Emperor title fair and square.