Rin Kagamine and Len Kagamine were created by Crypton Future Media as a singing synthesizer application software that could be used to cover music as well as create original songs. Their shared software included two separate voice banks, using the same singer's voice samples. This wasn't originally the plan, but because there was demand for another male Vocaloid while they were still creating Rin, it was decided that there would be both a male and a female voicebank included in the package. Because of this, Rin and Len are considered both to be mirror images and twins. They are both considered 14 years old in human years, but Rin is technically the older sibling since she was created first.
Rin's personality relies solely on the fans, but she is most commonly seen as the hyperactive and outgoing twin, who is also quite childish and bratty in her ways. Rin is also sometimes shown as a bit of a tsundere, as shown in some of her songs. Her character item is an orange, so it isn't hard to tell that she loves oranges above any other food.