Hatsune Miku
Hatsune Miku was the first Vocaloid developed by Crypton Future Media after they handled the release of the Yamaha vocal Meiko and Kaito. The name of "Hatsune Miku" was conceived soon after the Vocaloid 2 announcements, when an English and Japanese vocal were developed for the character. The intended design was delayed and later used for Megurine Luka instead. Miku was intended to be the first of a series of Vocaloids called the ""Character Vocal Series"", which included Kagamine Rin/Len and Megurine Luka. Each had a particular concept and vocal direction.
Kagamine Rin/Len
They received mixed results from producers because compared to Hatsune Miku, they were considered much harder vocals to use. Asami Shimoda noted that she often received comments from Vocaloid fans that Rin's voice echoes within their head and makes them feel dizzy, while Len's higher ranges were not favored by those not fond of the pair entirely.