Chapter 1057 reveals that Yamato will not join the Straw Hat Pirates crew. But what is the real reason behind Yamato’s surprising choice?
Yamato didn’t change idea about living Wano just because she wants to see the country. That’s clearly nonsense. Either Oda swiped off countless clues about Yamato joining the Straw Hats (I mean, she just declared that she was joining a few chapters ago) with a few badly written lines of dialogue or he is hiding something happened offscreen between Yamato and Luffy.
I think she is lying, and she is staying to protect Wano (by the way, she might feel it’s her duty because of what her father did to the country). Think about it: Momonosuke fought with an Admiral to show that he was able to defend the country. And proved that he had the courage, but not the strength.
Momonosuke and the Scabbards were losing badly against Green Bull: it was Shanks who made him escape.
Yamato saw this firsthand, and changed her mind about leaving the country. She also knows that, if she says this to Momonosuke, she will shame him: Momonosuke himself told her so. So she spoke with Luffy offscreen and explained that she wants to stay for as long as it’s needed. So Luffy gave Momonosuke a flag to protect the country, and Yamato stayed and lied in order to avoid offending Momonosuke: if he knew her real motives, he would literally order her to leave.
All this solves the problems with Yamato’s writing but also gives a reason for the introduction of Green Bull in Wano. Think about it, if Aramaki didn’t change Yamato’s mind about staying, he has not advanced the plot in the tiniest bit in the 3 chapters that he took centre stage. So Yamato is making a sacrifice, delaying her adventures for Wano’s sake. Luffy clearly stated in the last pages that, as soon as she wants to join, he’ll come back for her.