[Kaguya]

I doubt it's that to be honest because you see Len quite often, and Gakupo shows now and then. It's actually a misconception that Miku is a first generation vocaloid, when she is actually a second generation for example.

The pitch of her voice hurts my ears.

On another note Kaito is a first generation though was almost discontinued because very few liked his original voice; it was remade and thus we have the wonderful Kaito we have now.