By far the most controversial subject in any fandom is the coupling of characters in a series or in this case, technology. It should be noted that during Vocaloid2 era, no Official Vocaloid was a paired couple, a lover, a parent, or a sibling to one another. Most are given a brief description to describe what they are as a product but they are not officially given (or rather not announced to have) a role or pairing. The most notable example would be Kagamine Rin/Len, a female and male pair of voicebanks released together, and who therefore commonly end up singing songs together. The relationship between the two is often a heated fan debate. KEI, who illustrated them with the intention of being "mirrors" of each other that was given by Crypton, but Crypton has said no such thing themselves.
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On another note, the complementary voicebanks argument comes from the work of companies Zero-G and PowerFX, who have male vocaloids to go with their female vocaloids: Leon / Lola, Prima / Tonio, and Sweet Ann / Big Al. These examples show that they are meant to sing together because they are similar products, but they themselves were not created to be a couple in a romantic sense. Sometimes Meiko / Kaito and Gackpoid / Megpoid also receive this argument. The level of interest in fan pairings, however, caused Yuki Kaai's design to receive many complaints. Since she was intended to be the counterpart to Kiyoteru Hiyama, some fans reacted on the claim that the pair would become the subject of pedophilic artwork/songs because of the Lolicon culture.
It was not until December 2013 that the first official coupling was released. In April 2012, Aoki Lapis was released as a VOCALOID3. Much later, her elder sister was revealed to become an official VOCALOID, and was released in December 2013, therefore marking the pair as the first official siblings in the VOCALOID franchise. A few months later, the first twin pair of VOCALOIDs, anon and kanon, were released in March 2014, followed by the reveal of Yuezheng Ling, who had an older brother who was not a VOCALOID.
- Leon and Lola were considered "siblings" by fans and the media, and later a representative from Zero-G admitted sometimes they think of them as "siblings".
- Tonio was designed to be a partner to Prima.
- Kaito was designed to be a counterpart to Meiko. 
- Aoki Lapis and Merli are the first siblings.
- anon and kanon are the first twin siblings.
- Yuezheng Ling and Longya are the first "brother and sister" combination of siblings, however, only Ling is an official VOCALOID while Longya remains as a supporting character.
No exact confirmation on any of the products has ever been assigned and no relationship between Vocaloids has ever been confirmed. However, Sonika once called all older Zero-G Vocaloids as her older siblings, but referred to Prima as "auntie Prima". She also considers all other Vocaloids her cousins.
- ↑ [http://vocaloidotaku.net/index.php?/topic/3403-questions-targeted-at-zero-gpowerfx/page__st__15__p__76264&#entry76264 Note
- ↑ link
- ↑ http://internet.watch.impress.co.jp/cda/president/2008/05/12/19477.html
- ↑ http://dic.nicovideo.jp/a/vocaloid%E9%81%94%E3%81%AE%E3%82%AB%E3%83%83%E3%83%97%E3%83%AA%E3%83%B3%E3%82%B0