Yowane Haku's whose name means "speaking weak words" ('speaking' here has connotation of a pent-up sigh or muttering.) She gets her start from a thread in 2chan where a user commented: "I bought Hatsune Miku for her cute looks but no matter how hard I try, I can't get anything better than mechanical sounding songs out of her."
An artist (Caffein) took this idea and ran with it to create Yowane Haku, an alter ego of all unskilled users of Hatsune Miku. Haku is a sad Vocaloid who just can't sing as well as Miku. She is always crying because of this and vents her frustration by drinking. Her image colors are silver and purple. Her character item is a sake bottle. She's also noted for her curvier figure compared to Miku and has a decent following.
This created a trend of users labeling badly-made Vocaloid songs with voyakiloid, or "shouting vocaloid." It is implied that it was Haku who sung those songs, and not Miku.