That's certainly quite the theory you have there, with some rather interesting information that definitely gives a reason to believe. As a Tekken fan, I'm delighted to see the comparison!
I'll try to break a few things down based upon what I've learned over the years. From the character style perspective, Toshinobu Kondo's art style has constantly changed. As a result Umihara Kawase has never stuck with an exact art style. This becomes really apparent by how both versions of Shun have different styles of Kawase on the cover. The only real change to Umi across the games has been an increase being done to her bust. Making it either far greater, or more noticeable.
While the tomboy stuff is quite interesting, Kawase, to be frank, doesn't have a personality or really any character traits. The only thing we've known about her is that she's a wandering chef, with the only character elements being added into Fresh and Blade Strangers. From there, it would be anyones guess if any of the character traits featured in either of those games are just side story elements, or actual character traits. KawaseFan could most likely speak on that far more accurately than I could, as I might very easily be missing something that may of been mentioned.
In terms of the bikini itself, I'd venture to guess that has something more so to do with a way of offering fan service for marketable audience appeal. Umihara Kawase games have made a larger focus in recent iterations on Umi's appeal. Like how her midriff is exposed in Fresh.
Interesting comparisons all the same!
