History of Vocaloid2 : Rin/Len Kagamine

Crypton Future Media intended to project a Vocaloid with low-teen girl's voice at first following Miku Hatsune, but there was a demand for a boy's voice, so Crypton Future Media hired a voice actress who can produce both sounds. The package included two voice banks: one for Rin and another for Len, both provided by the seiyū Asami Shimoda (下田 麻美 Shimoda Asami). Despite the double voice banks, the package was priced at the same price as Miku Hatsune.

At the time of recording in 2007, their names had not yet been decided. Their given names of Rin and Len were said to be based on "right" and "left" according to Asami Shimoda. She later confirmed in a interview after the Append release, the name itself was based on the names of Ken and Rin from Hokuto no Ken, the names were deprived from a discussion about names that were familiar from her childhood. According to Asami Shimoda, Len's voice was achieved by singing within her chest, while Rin's at the top of her head.

From :
http://vocaloid.wikia.com/wiki/Kagamine_Rin/Len