>>282416340
>Whirlwind made sense for Himari since hit-and-run speed blitzing Yachi was their best shot at winning.
Saying that implies the transformation process has some kind of conscious ability to mold itself to the needs of the users, even before the users themselves realize what kind of power they need. Since it's not like Yuuki and Himari had already come up with a plan to defeat Yachiho with speed and the form manifested that way in response, they didn't even know Yuuki would have different forms the first time Himari tried copying slave. They were just trying out fighting together for the first time and happened to discover Yuuki transformed differently for a different rider.

It's really not clear what determines how each new form looks and what it can do, aside from it sometimes having synergy with the riders powers, or somehow being molded by the riders personality and mannerisms. But in reality it's probably explained the same way as almost everything else in this manga, Takahiro makes it up as he goes based on what seems cool at the moment.