

Haku started out as representing any Miku song that was bad, so had no special adjustments made at all. Except there is no difference, Haku's voice and Miku's were one and the same. This reminds me of an old ancient argument back in Vocaloid2's days wherein someone once said they hated Miku but loved Haku's voice. You can't just magic a voice for the pair because of all this. There are some 70+ Vocaloids now and Haku and Neru aren't really taken in the same way they was in the past.

I'm not hating the pair here, so don't take it as that. This is why are considered "deviants" of Miku's design. As I said, their not even Vocaloids, they don't have voicebanks and Yamaha won't acknowledge them. You may listen to Miku and Rin in different ways, but they are still Rin and Miku, seriously, there is no dedicated vocal for Neru and Haku therefore you can't really do a "cover song" as its still Rin and Miku.
