Looking at Yamato as a character, Oda-sensei introduced them in a very specific way that directed him toward Luffy. From the fact that Luffy was the person Yamato was waiting for to Yamato’s expression of wanting to travel aboard Luffy’s ship. There wasn’t much hesitation in Yamato’s mind about leaving with the Straw Hat Pirates once Wano was liberated and its borders opened. In order to make a case for Yamato remaining in Wano, the narrative would need to entrench Yamato and his desires within the plot threads of Wano but as at chapter 1049, that has not yet happened (potentially it still could). Rather, Yamato leaving with Luffy has only been reinforced.
How Yamato was introduced:
Let’s look at how Yamato was introduced. He hid himself from the Beast Pirates during that day in order to wait for Luffy to arrive so that he could meet him. The fact that Yamato specifically searched for Luffy and intervened in Luffy’s fight against Ulti conveys how focused Yamato was on Luffy. This reality became further apparent when Yamato declared he would fight on Luffy’s side. At this point, Yamato’s primary interest was Luffy, whom Yamato believed would stop Kaido. Yamato even told Luffy that he had no issue fighting against his father when Luffy inquired if he was okay with his father being defeated. In order for Yamato’s story to align with Momonosuke and Wano, the primary focus within Yamato’s mind would need to switch from Luffy to Wano.
Going forward, Yamato rejected Kaido’s intention to make him shogun and became further indebted to Luffy when his explosive bindings were removed and Luffy shared with him the information that Momonosuke and the Akazaya Samurai were alive. Yamato guided Luffy to the Performance Floor where Momonosuke was chained. And this is where a very important moment conveying both Oda-sensei and Luffy’s impression of Yamato took place. When Yamato attempted to engage his father, Luffy stopped him and stated that Yamato and him weren’t the ones who needed to start the War. The ones who needed to initiate the War were the representatives of Wano i.e. the Akazaya Samurai. In Luffy’s eyes, Yamato was not a representation or reflection of Wano. Yamato was like him, an outsider. He was not an agent of Wano. Oda-sensei, at this point, clearly set up Yamato to be separate from those who truly belonged to Wano.
How Yamato and Momonosuke’s relationship developed:
Once the Akazaya Samurai attacked Kaido and began the war, Yamato could have rushed to Momonosuke to help save him but instead, Yamato remained beside Luffy while facing Ulti. Yamato declared he would become Oden on this day and that he stands in opposition to his father. Yamato also heard Luffy declare his intention to defeat Kaido, the Beast Pirates, Big Mom and Orochi confirming Yamato’s belief that Luffy will be the one to save Wano. If at this point in time, Momonosuke was Yamato’s primary focus, Yamato would have acted to save him but instead, what we have is Yamato only taking on the responsibility of protecting Momonosuke AFTER ordered to by Luffy. Yamato’s primary focus remained Luffy with Momonosuke becoming his secondary focus. When Yamato saves Momonosuke, he mentions that he would die for Momonosuke. There is no lies in Yamato’s words but this moment did not push Momonosuke to be Yamato’s primary focus substantiated by Yamato still leaving Momonosuke to assist Luffy against Kaido and a second time leaving Momonosuke to stop the Onigashima bombs from exploding. These moments convey how Momonosuke continued to remain secondary to Luffy in Yamato’s mind.
The story Yamato shared with Momonosuke and Shinobu about his past conveyed how Luffy became important to Yamato after meeting Ace with Yamato only realizing Luffy’s relevance AFTER Ace died. It also presented to readers how meaningful being free and sailing the seas were to Yamato. There is also the fact that Yamato now knows what Luffy’s dream beyond being Pirate King is. A dream very intimate to Luffy as he expressed it in front of his brothers in the past. Additionally, chapter 999 sets up Yamato visiting Ace’s grave once Kaido is defeated and Wano’s borders are opened. There will definitely be payoff to the set up established.
Arguably, the flashbacks also indirectly convey Yamato’s 3D2Y moment. The moment Yamato lost Ace was the moment he gained Luffy. Through the grief and sorrow, Yamato’s familiarity with Oden’s words and the story Ace shared with him unlocked a connection Yamato never noticed before. Luffy was the one Oden had been waiting for and desired to open Wano’s borders for. When Yamato met Ace, he was okay with leaving Wano on Ace’s ship if he could but once Yamato made the connection between Luffy and Joy Boy, he realized how important it was to open Wano’s borders. Freedom and adventure were always Yamato’s focus but saving Wano and opening its borders were never his primary concerns when he met Ace. It was Luffy who established in Yamato’s mind the need to have Wano’s borders opened. This is why Yamato has been waiting for Luffy and unlike how his meeting with Ace played out, Yamato is now aware of the importance of defeating Kaido and liberating the nation before leaving.
Yamato abandoning Momonosuke to help Luffy fight Kaido establishes how important Luffy is to Yamato relative to Momonosuke. If Momonosuke was Yamato’s primary concern, Yamato would not have left Momonosuke vulnerable. Clearly at this point, Luffy is still Yamato’s primary focus. Yamato even mentions to Kaido without ambiguity and uncertainty or any other room for interpretation that he plans to LEAVE Wano WITH Luffy after Kaido is defeated. Those words are clear. No hesitation. No doubt. Only resolve.
Yamato goes on to further state that Luffy is the one who Oden was waiting for i.e. Joy Boy. This expression clearly establishes why Yamato is ready to fight for Luffy. Thanks to reading Oden’s journal and treating it as his Bible, Joy Boy (Luffy) became a figure of salvation within Yamato’s mind for Wano and the world. When Ace died, it was the connection Yamato made between Luffy and Joy Boy that likely inspired him to continue fighting through the grief and powerlessness (similar to what happened to Sabo). Through Yamato’s battle with Kaido, the theme of liberation became a focal point to his character. From his flashbacks, to the events in the present day, Yamato continuously challenged Kaido and refused to yield. Freeing the world is a story Joy Boy is destined for and Yamato very much aligns with this in regards to his past and the story playing out in the Wano Kuni Arc.
The main reason driving Yamato to fight Kaido was not to defeat Kaido and save Wano but to hold Kaido back until Luffy returns to challenge him again. Yamato even states this when Kaido expresses that Yamato cannot defeat him. Yamato knows he cannot defeat Kaido alone yet he still chooses to fight and this is because of Luffy. When Luffy does return, Oda-sensei expresses how well Luffy and Yamato synergise through their joint attack against Kaido. This is one of Yamato’s best panels illustrated by Oda-sensei. Yamato left Kaido to Luffy when he returned and after seeing Luffy and Kaido split the sky like Oden described Roger and Whitbeard doing, Yamato had more confirmation that Luffy truly is the one to defeat Kaido. What Oden began to see in Roger which played a part in Oden agreeing to travel with Roger over Whitebeard, Yamato is seeing firsthand in Luffy.
Yamato’s belief in Momonosuke:
Moving ahead, once again Yamato does not proactively leave with Momonosuke, he only does so when Luffy request Yamato to help Momonosuke with stopping Onigashima from falling on the Flower Capital. During Yamato guiding Momonosuke, he does not scold Momonosuke for being a coward (because Yamato doesn’t believe he is) but he does yell at Momonosuke for not believing in Luffy. Yamato believes Momonosuke is capable of saving Wano which is why Yamato left Momonosuke alone to deal with the Onigashima explosives. If Yamato truly felt Momonosuke was not ready and needed more of his advice, Yamato would have passed the responsibility of stopping the Onigashima bombs from exploding onto someone else and returned to help Momonosuke. That clearly did not happen. Only after the Onigashima bombs were dealt with did Yamato and Momonosuke reunite and this wasn’t by intention but by circumstance. Once the Wano Kuni Arc concludes after Momonosuke carries Onigashima on his back, Yamato will have little reason to remain in Wano over leaving with Luffy and helping him.
Turning to scene where Momonosuke questions whether he is a coward, he expressed his frustration to Yamato at not knowing what his father discovered on Laugh Tale. Momonosuke’s desires of wanting to know the contents of the torn out page from Oden’s journal was conveyed to Yamato. This scene doesn’t express to Yamato that Momonosuke isn’t ready so he needs to remain in Wano but rather it gives Yamato more of reason to leave. Momonosuke wants to know the secrets of Laugh Tale and he confided this in Yamato. Not Luffy, Kin’emon or Robin but the child of Kaido. Yamato has a genuine reason to travel to Laugh Tale and help alleviate Momonosuke’s confusion. Momonosuke is destined to guide the world to the Dawn which is a plot thread set up to conclude after the Straw Hat Pirates reach Laugh Tale. And due to Momonosuke having confided in Yamato, it will be him conveying the secrets of Laugh Tale to Momonosuke.
Through Momonosuke, who heard the voice of Zunesha, Yamato received confirmation that Luffy was Joy Boy. Yamato even queried Momonosuke on this. When someone asks questions about a subject matter, it usually means they are interested in the topic and wish to know more. Yamato witnessing Luffy in Gear Five fighting Kaido as an equal gave Yamato more motivation to help him by requesting Momonosuke to keep Onigashima from falling. Yamato continues to believe in Momonosuke’s ability and the reality that Momonosuke is ready to carry the nation and the future of the world on his back.
Yamato continues to operate with Luffy as his primary focus. And at no time during the arc as at chapter 1049, has Oda-sensei had Yamato place Momonosuke as a concern above Luffy. Momonosuke has always been a secondary focus to Yamato. To Yamato, Luffy has been the one primarily driving his choices, actions and desires.
These are my thoughts on Yamato and how the plot is influencing the direction he is being developed in. The Wano Arc isn’t just treating Yamato as a main character but the whole manga is supporting his story; from Ace, to Oden, to Luffy/Joy Boy, to Wano, to Kaido, to Xebec D. Rocks (likely betrayed Kaido), to Blackbeard (his actions resulted in Ace’s death), to the Ancient Giants/Oni, to the Dawn, to the themes of liberation, freedom, romance and coexistence. The manga itself is highlighting Yamato as unique and important. Every single expected future arc has story threads related to Yamato.