Kagamine Len is Kagamine Rin's twin brother. He was announced after her, making him the younger sibling. Due to high demands for a male vocaloid, wat-san decided to release a male voice alongside Rin's release. Since Asami Shimoda did both voices, one person would be a 'mirror projected shape', as if Rin & Len were (made by) the same person, though most of the fans refer to them as 'the twins'. Kagamine stands for Kagami(鏡) = Mirror and Ne(音) = Sound, with 'Len' from the word "Left" and 'Rin' from "Right".
The announcement of Len was indeed a surprise to the public, since Rin had already been presented a few days before his character would be shown. His demo song was L-Side (There was also a low key version). Just as Rin, Len was described to have a powerful voice. They were released on December 27th, 2007.
Although quite a bit of feedback on Len's voice wasn't initially so good, the real deal came with the release date and some songs that were uploaded to nico. One of the first songs from Len (and most-welcomed) was Tori no Uta's cover arrangement:
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