Crypton Future Media intended to produce a VOCALOID with a young teen girl's voice at first following Hatsune Miku, but there was also demand for a boy's voice, so Crypton Future Media hired a voice actress who could produce both. Kagamine Rin was the first voicebank developed and introduced to the VOCALOID consumer market on November 8, 2007. However, the package included two voicebanks: one for Rin and another for Len, both provided by the seiyūAsami Shimoda (下田 麻美 Shimoda Asami), Kagamine Len was unveiled after Kagamine Rin on December 3, 2007. The package was the same price as Hatsune Miku, even though there were two voicebanks in the box.
According to Asami Shimoda, Len's voice was achieved by singing within her chest, while Rin's at the top of her head.
The Kagamine's were released as a more advanced VOCALOID package than Hatsune Miku, yet carried on the concept ideas, this time by releasing a package containing 2 voicebanks for contrasting male and female results. Since they were sold together as a pair at the same price as Hatsune Miku, the Kagamine package was considered a bargain. Their vocal results are obtained from voice acting, therefore do not intentionally produce professional singing results.
Both Rin and Len are "power" vocal types.
Due to its flexibility the vocals could to be used for a high variety of songs.
Compared to Hatsune Miku, the Kagamine package was overall ranked as a lesser quality product.
This particular package had a reputation for being difficult to work with.
Crypton Future Media received many complaints from producers who had high expectations after Hatsune Miku's release, due to the amount of issues with Rin and Len's vocals.
After the release of "Act2", this software became referred to as "Act1" by both Crypton Future Media and Vocaloid fans.
The default voice for this package is "Kagamine Rin" and users have to manually select the Kagamine Len vocal.
Len is the male counterpart to the Rin vocal and like her is intended to sound "deep" in tone. He is a young boy sounding singer and the vocal range is in the Soprano scale.
His voice has a slight "lisp" sound which separates his vocals overall from Rin's.
He can harmonize with Rin relatively easier than most Vocaloids due to having similar vocal capabilities.
His vocal range capabilities are also slightly larger than Rin.
At a known optimum vocal range of #C5 Kagamine Len has the highest male vocal within the Vocaloid 2 range, however, take in mind that he was voiced by a female vocalist.
Compared to the Rin vocal, Len is often weaker on higher notes
Len is often considered more difficult to work with than his counterpart, Rin. 
His vowel sounds are choppy and overall lacked smoothness with a number of vowel related sounds being absent.
He is better at slower songs compared to Rin.
A particular noted weakness of Len's voicebank is that he is limited in his capabilities of matching masculine vocal tones, owed to his voice provider being female. Therefore, he cannot produce a satisfactory low masculine vocal tone.
According to Asami Shimoda, there are some complaints towards the Vocaloid within the Japanese fandom on how they sound. Some say Len's high vocals do not appeal to those not fond of the pair.