Mizuka Nagamori

Mizuka is Kōhei's classmate and childhood friend. She is constantly sighing, being made to go along with Kōhei's meaningless jokes. She takes unusually good care of him such as going to his house every morning to wake him up. For clubs, she helps around the orchestra club and plays the cello. She likes milk so much that she can even drink it while eating rice. Mizuka is a cat-lover and has several of them at her house. She tends to add "~da yo" and "~mon" at the end of her sentences and at one point of the story she gets called a "Dayomonian", but only once. She worries about Kōhei, wondering if he could suddenly get a girlfriend. Later, there is a motif where her childhood self becomes a guide for the main character in the Eternal World. Voiced by: Mayumi Iizuka (PS), Yūko Minaguchi (drama CD), Ayako Kawasumi (all-ages OVA), Ayaka Kimura (PC full-voice).

From Wikipedia.