No one. That's her voice.

Hatsune Miku is a vocaloid. A voicebank created with the software VOCALOID. Basically a voice in a box. And then you have the mascot Hatsune Miku that is that girl with long twintails.

Miku, just like all the other vocaloids is an musical instrument. You can create a song, then use them to generate vocals to mix with your song. That's why you have plenty of songs where the same vocaloid can sound really diferent, because it depends of who's using them. Vocaloids give the opportunity for many talented composers to make their songs the way they want including the vocals.

Here's an example of Miku bein used together with another vocaloid voicebank, KAITO (guy with blue hair) and he's from the same company that created Miku: https://www.youtube.com/watch?v=dXqWiAP2MO4#t=50

This one is of only Miku and you can hear how diferent she can sound depending of the tuning and whoever is using her: https://www.youtube.com/watch?v=llZOAoEI6kY

Yes, Saki Fujita's voice was used to create the voicebank Hatsune Miku, but their voice don't sound the same. So even thought Saki was the voice provider for the voicebank, Miku's voice is Miku's voice.

Here's an duet between Miku and Saki: https://www.youtube.com/watch?v=zcIxD4pP6N4

Miku is a digital singer. Her voice is computer-generated and is really her own.

The underlying phoneme samples were provided by actress Saki Fujita, but a lot of audio engineering and software algorithms are used to create Miku's voice and many people were involved in that. To put it all down to Saki Fujita is crazy. She does not and cannot sing like Miku. She can sort of do a Miku impression, but it isn't convincing.

Miku's voicebank was recorded by Japanese voice actress Fujita Saki.

http://en.wikipedia.org/wiki/Saki_Fujita

saki fujita lent her voice