In my eyes, Yamato is arguably one of the most uninspiring characters that Oda has come up with thus far. I will give reasons as to why I feel this way so if you disagree with me bring valid arguments to the table.
I disagree on almost every point here, but that seems to be because we have different expectations for the character moving forwards.
1. Yamato's design.
Females in OP usually follow the same pattern in terms of their design. This isn't something that usually bothers me but the issue here is that it feels very unconvincing that Yamato is supposed to be Kaido's child because of her average design. Comparatively speaking, most of Big Mom's daughters' have idiosyncratic designs that add to the credibility of them being BM's children. Yamato just doesn't have that.
I mean, fair, she does still have Oda's generic female design as the basis for her face despite the unusually harsh eyebrows, horns, color scheme and outfit.
But on the other hand, I will still suggest that its unique, because she's still the very first character with such a "Pretty" design, being portrayed as such a physical powerhouse.
2. Yamato's relationship to Luffy & the alliance.
Ever since her introduction she is basically just a plot device there to help Luffy and the others to fight against Kaido.
You cant really be calling her a plot device when she's accomplished so little so far, can you? I mean, she took out a number and prevented a conclusion to Luffy vs Ulti. And she's be trying to keep momonosuke safe.
She's definitely most interesting due to her character and her relationships.
There is no attempt for us to feel like her motivation for killing her own father is realistic.
You say that, but we haven't even reached the point of the arc where we get to see them interact, let alone a flashback between the two of them.
Every good one piece villain needs at least one final act of irredeemability before Luffy can pummel them with 100%. Like Luffy finding Nami's room in Arlong Park. Like the Chopper flashback in drum Island. Like Crocodile turning on Miss All Sunday in Alabasta. Like Doflamingo making Rebecca kill Viola.
Just how bad Kaido treats even his own daughter, might be that narrative punch needed to ultimately sell Kaido's defeat.
There is no gradual build-up with her motives. It's literally just her meeting Luffy and ''hey, I'm going to help you defeat my father'' or ''hey, we just met but let me on your crew''.
You're assuming that this buildup hasn't already happened before the two of them even met. Yamato knew about Luffy for years; we've already seen that. We know that a resolution to help a Pirate defeat her father had been present even as far back as when Ace first came to Wano.
Most of the members within the alliance didn't just immediately join but we see them deciding to fight against Kaido and Orochi through their own personal convictions which makes it feel realistic and earned. Yamato is literally fighting against her own family but the credibility behind it feels ridiculous.
Yamato, at very least, has several of the same reasons to fight as the scabbards do, since she was there and witnessed the execution. And she knew Oden as if in the flesh, thanks to his diary. AND I think again, it's pretty obvious that we don't fully understand how bad Kaido is to her yet.
3. Yamato's role in general.
It's very common for Oda to introduce a lot of characters within an arc and that's fine but the issue is that a large percent of those characters end up doing almost nothing relevant. This brings up the question why some of these characters were even brought into the story in the first place. This is were Yamato comes into place. What has she done so far? Yamato has been running with Momonosuke and Shinobu fighting against fodder and pointless shenanigans even though it has already been established that Momo isn't going to die. Do we really need a side-plot like this in the semi- endgame of OP?
The side plot is us getting to better know Yamato (we already got a connection to Ace through it). And for Yamato to better know Momo, farther giving her reason to oppose her father in the big scene that will surely come and act as a turning point this arc.
Oda could have simply skipped all of those pages and focussed on the Tobbi Roppo and the calamites which have been on the side lines for a very long time.
But then the very most important piece in this, the final confrontation with Kaido, would suffer. You understand that right? Yamato is primed up to be the very most important and richly symbolic piece of the puzzle in all of the Wano arc. It's like you're complaining that a bullet is useless, despite the gun not even being fired yet.