About 5 months after Hatsune Miku(CV01*) was released Crypton Future Media released Kagamine Rin & Len(CV02) in December 27, 2007 a japanese female and a male Vocaloid, their voices were provided by Asami Shimoda a japanese seiyū and actress, which isn't uncommon since young boys are often dubbed by adult females.
People often think Rin & Len were twins or just siblings, which is not true, according to their names (Kagami - Mirror, Ne - Sound, Rin which does not mean right but stands for it and Len for left) Rin and Len are mirror reflections of eachother.
*CV = consonant-vowel, it means that the voicebank is based on consonants followed by a vowel or single vowels:
ki ka ke ko ku ha he hi ho hu sa se si so su etc. just like japanese syllables (see here)
Kagamine Rin & Len - Adolescence
Illustrations by 田村ヒロ
Title/Logo by タイトルロゴ
Lyrics by orange
Animation by シグナルP