As we all know now, Yamato had met “Fire Fist” Ace at one point before the Straw Hats arrival at Wano. This meeting was filled with insight into who Yamato is, and her relationship with Ace. There has been alot of Yamato-hate since her debut however I feel this hate is grossly misplaced.
Yamato’s Self Stylization of Identify (Kozuki Oden) is just that, an Alias given to herself by herself because she has a purpose she wants to fullfill.
As highlighted in the image below, Yamato had been running the only active effort to resist Kaido for years because to her knowledge the Akazaya Nine were all dead. She had ALWAYS been an Ally to the crew – years before she even met them.
Meeting Ace started her Active Rebellion against Kaido. Ace told to her to stop letting Kaido chain her heart.
And Luffy freed her physically.
Here we learn the reason for her self stylization. Yamato’s Alias of Oden was symbolic of Freedom.
Yamato’s Desire for Freedom was so great that as a child she unlocked her inborn Conqueror’s Haki.
Yamato has repeatedly stated she will be on the Straw Hat crew and go to sea with Luffy. And not once has Luffy rejected her claim.
Even now in Chapter 1051 Yamato was shown to be accepted by the entire crew:
And to those who claim she will never be allowed on the ship because she knows where Laugh Tale is or what the One Piece is.. well, Momonosuke confirmed somebody (assumed to be Oden) removed every important detail about the voyage he was on to prevent the wrong hands from learning anything before Wano opened its borders.
All in all, Ace told Yamato to seek Luffy if he never returned. This implies Ace made a Promise across time for Yamato to join the Straw Hat Crew under Luffy – A Promise Luffy would never fail to keep for Ace’s Sake.