Koyomi is voiced by upcoming actress Rie Tanaka. You can visit her site here: http://rie.bird.to for more information and pictures.

Rie's other voice work include the following:

"Chobits" (2002) TV Series .... Chii/Black Chii
"Dual! Paralle lunlun monogatari" (1999) TV Series .... Mitsuki Sanada

This information was acquired from the Internet Movie Database (IMDb)