The second release of Crypton's Vocaloid 2 came in a two for one pack.  Kagamine Rin and Kagamine Len, both voiced by the same artist, were released together, intending to be one 14-year old girl and one 14-year old boy.  Their original design was supposed to be a mirror effect, as Kagami means Mirror in Japanese, however in most canon stories, they have been known to be the Kagamine Twins.  Like Miku before them, their outfits were designed to reflect that of a high school uniform.  Rin's signature food is usually an orange, while Len's tends to be a banana.  Len can sometimes be seen with a Keytar in his hands, while Rin holds the microphone.  Also around them tends to be an ever-present Road Roller, which is the Japanese designation of a steamroller.

Rin and Len often are found doing duets, as they were designed to do, and their canon created the Country of Evil saga, where Rin is a selfish and destructive ruler, while her brother was raised to be her loyal servant, as well as a common joke "Midori wa teki ga!" pits Miku as their sworn rivals.  However, while they do have many great works together, some of the best have come from their solo performances.  Rin starred as an adult in Roshin Yukai: Meltdown, which has been a smash hit ever since its release.

They have made guest appearances in SEGA's Project DIVA.  Crypton released a supplementary library to fix some glaring flaws in the original at no charge to the original purchasers, known as Rin and Len Act 2.