The message is clear: kouji intends to show Fubuki being herself isn't vanilla at all.
Was she ever vanilla in stats and looks in the first place?
I never thought it was fair that "vanilla" is equated with blandness in the first place. It's one of the most popular and sought-after flavors in the world. Vanilla pods are the second-most-valuable spice on Earth, capable of being cultivated, at great effort and expense, in only a handful of places. Only saffron costs more per unit mass, and that's only because it's virtually impossible to cultivate; vanilla is by far the more popular of the two. That all seems pretty special to me. And yet paradoxically, thanks to its massive popularity, it's entered the English vernacular as a synonym for bland, boring, unimaginative, and ordinary - which completely overlooks the simple fact that you can find it everywhere because virtually everybody wants it.
So yeah, Fubuki's vanilla. Because vanilla is awesome. :)
