Spoilering because it's not really about the current design topic, but I wanted to talk about Yuki's popularity a bit.
I don't have much to add that hasn't already been said, but on Kaai Yuki, you have to remember she released 15 years ago. She was not at all popular for that entire 15 years - in fact, her surge in popularity is actually pretty recent aside from having one single hit song (Ikanaide) that people knew her from for a decade before Inabakumori came along and now a ton of producers use her.
So saying things like Yuki has a cute design or a unique voice may be true, but they're not really the reason she's popular. Because if it were, then she would've been this popular for the entire 15 years she's existed with tons of producers using her when it's really only been the case for the past few years. That being said, I do think Yuki has always been one of the easier Vocaloids to stumble across, and I think more casual listeners Might have been able to recognize her easier than others (before her surge in popularity, I mean), but really I think her current popularity just stems from Inabakumori's music going viral (which in itself is crazy impressive because again, Yuki songs were not regularly gaining millions of views before him and could even be considered obscure) and more producers picking her up from that alone.
So saying things like Yuki has a cute design or a unique voice may be true, but they're not really the reason she's popular. Because if it were, then she would've been this popular for the entire 15 years she's existed with tons of producers using her when it's really only been the case for the past few years. That being said, I do think Yuki has always been one of the easier Vocaloids to stumble across, and I think more casual listeners Might have been able to recognize her easier than others (before her surge in popularity, I mean), but really I think her current popularity just stems from Inabakumori's music going viral (which in itself is crazy impressive because again, Yuki songs were not regularly gaining millions of views before him and could even be considered obscure) and more producers picking her up from that alone.