Kasane Teto is more of a special case whereas she's not a Vocaloid but instead UTAU. UTAU is a different program that's similar to Vocaloid but different companys and different programs. Teto was released in Apirl 2008 and is supposely 31 and 15.5 in chimera years. Teto was originally created as a troll Vocaloid and was never going to be a serious creation but only for April fools. Nobuyo Oyama provided the voice for Teto and the Teto project continued even after the troll campaign. She started to gain popularity after a few years allowing her to be in multiple games with multiple popular Vocaloids such as Miku. Her creator is known as Twindrill and is known as one of the biggest reasons why UTAU became popular due to her fame. Teto is the first UTAU with a recorded voicebank and is the first VIPPERloid. She's also the first UTAU character to appear in the Project DIVA video games.
