"Dark King" Rayleigh was Roger's first mate and one of the strongest characters in the One Piece universe. He was also directly responsible for Luffy's training during the two-year time skip. Within that time, Rayleigh turned his student from a reckless child into a warrior capable of toppling emperors.
However, there are many other individuals that Rayleigh would have trained in Luffy's place if given the time and opportunity. By identifying them, it becomes easier to gauge Rayleigh's priorities, strengths, and the values he appreciates in a potential student.
10 Ace Was The Son Of Rayleigh's Captain
There are a number of considerations making Ace an attractive prospect for Rayleigh to train. In addition to being Roger's son, he was also the prophesied future king of the pirates. Further, given that Ace is relatively inexperienced in armament and observation haki, he would benefit heavily from Rayleigh's lessons.
The only problematic aspect is his attitude. Since Ace tries to erase his father's influence from his life, he might not be as receptive to Rayleigh's training on account of his background. However, accepting it would've given him the experience necessary to defeat Blackbeard in their fateful encounter.
9 Buggy Was A Former Member Of Rayleigh's Crew & Has Since Become An Emperor
Buggy was a member of Roger's crew as a boy and only missed seeing the One Piece due to an inconvenient case of cabin fever. Given this stroke of misfortune, Rayleigh would feel greater pity for him than the average pirate.
Considering that Buggy is now a yonko — one of the Four Emperors — Rayleigh would be even more inclined to train him. If he could help Buggy become as strong as his title demands, his former crewmate might not have to rely on luck anymore.
8 Kin'emon Would Put Rayleigh's Training To Good Use
"Fox Fire" Kin'emon would be an ideal candidate for Rayleigh to train. Not only did he show tremendous growth under Yasuie and Oden, but he is also willing to give his life for a worthy cause. Humble and kind, the only issue Rayleigh might encounter when training Kin'emon is his age.
However, the rewards would be well worth the downsides. Not only could Kin'emon pass down Rayleigh's lessons to Oden's other retainers, he could also teach Momonosuke. This would ensure that the Land of Wano remains well protected from future threats and those unworthy of being the next pirate king.
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Rayleigh Would Hone Usopp's Existing Toolkit
Usopp may not be particularly courageous, but Rayleigh would still train him as Luffy's ally. Fortunately, the sniper has a number of assets to work with and develop. For example, his observation haki is so legendary that he could even snipe opponents obscured by walls and distance.
Should Rayleigh teach Usopp armament haki, it would benefit him in a number of ways. Not only would it protect the sniper in close-range fighting, it could even enhance the lethality of his slingshot.
6 Rayleigh Could Make Shanks Even Greater
As another original member of Rogers' crew, Shanks' proximity to Rayleigh would make him a more compatible trainee. Like Buggy, he also did not see the One Piece despite the extensive time spent aboard Roger's ship.
Should Rayleigh have trained Shanks from childhood onward, it's likely that the man would have become even stronger than he is today. Though Rayleigh and Shanks are now closer to equals, the latter would still benefit from his wisdom and vast experience. As an emperor of the sea, Shanks cannot afford to ignore what Rayleigh might have to say.
5 Rayleigh Could Help Zoro With His Conqueror's Haki
Though seldom using it, Zoro cannot be overlooked as a man who possesses conqueror's haki. Immediately, this should render him a valuable student for Rayleigh, as Zoro can profit from the full extent of the man's teachings. Additionally, enhancing his observation and armament haki further would increase Zoro's striking power exponentially.
The only disadvantage to training with Rayleigh over Mihawk is that the latter capitalizes on sword-based techniques, whereas the former's fighting style is more general. Ultimately, Zoro does not simply want to be a great fighter; he only wants to become the world's greatest swordsman.
4 Bon Clay Would Combine Rayleigh's Training With His Unorthodox Techniques
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Bon Clay may not have a formal relationship with Rayleigh, though he's proven himself time and again. Whether enabling Luffy's freedom at the cost of his own or standing up to Magellan, Rayleigh would observe Clay's actions and agree that he's worthy of training.
Since Clay does not fight traditionally, he would be able to combine Rayleigh's techniques with his own in order to invent a completely original fighting style. When combined when armament haki, he would be formidable.
3 Oden Would Thrive Under's Rayleigh's Instruction
Having fought against Oden personally, Rayleigh understands the man's power and untapped potential. For all of Oden's great feats, he has never been formally trained. Instead, he relies on raw strength, speed, and instinct in order to win his battles.
Should Rayleigh have helped refine the Oden's skills and haki, he would have been a legend like no other. With adequate armament or observation haki, Oden may have been able to defeat Kaido in their climactic encounter over twenty years ago.
2 Marco & Rayleigh Have A Special Relationship
Marco "the Phoenix" has a special relationship with Rayleigh. Both are commanders in their respective crews and relics from a previous era. As a result, he would be compatible to train, regardless of their captain's preexisting differences.
While Marco has already mastered most of the techniques that Rayleigh has to offer, they can clearly be refined further. This was seen through the difference both men had when fighting Admiral Kizaru. Should Marco increase his basic skills under Rayleigh's watchful eye, he may one day reclaim the Whitebeard pirates' status as emperors of the sea.
1 Sabo Was As Close To Ace As Luffy
Considering their relationship as "adoptive" brothers, Sabo was as close to Ace as Luffy. Given Rayleigh's aforementioned connections, this would immediately endear the two to one another. Even if Rayleigh could not train Ace as the next king of the pirates, Sabo would make for a worthy substitute.
As second in command of the Revolutionary Army, Rayleigh would also appreciate Sabo's efforts on the global stage. Since he's witnessed the corruption of the Celestial Dragons firsthand, he'd assist Sabo's endeavors to end it. Additionally, Sabo's youth ensures that he will make the most of Rayleigh's lessons for decades to come.