Yuki Kaai

Kaai Yuki is a Japanese VOCALOID developed and distributed by AH-Software Co. Ltd., and was initially released in December 2009 for the VOCALOID2 engine. Yuki's vocals are based on an elementary school student. Since the voice provider was a young fourth grade girl at the time, her identity was never revealed for legal reasons. Her voice provider was able to speak about Yuki for the character's 10th anniversary, but she still remains anonymous. She was the first female VOCALOID whose voice samples were not produced from an adult.

She was updated to the VOCALOID4 engine in October 2015 and used a new voice provider for the growl components. This was due to her initial provider maturing, making re-sampling problematic.

(Source: VOCALOID Synthesizer Wiki)