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Character:

Hikari's roommate. She is a somewhat forceful girl who also seems to look out for Hikari, who she is in fact in love with.
Yaya is good friends with Hikari though they've only known each other for about a year once the story begins. Yaya got Hikari to join the St. Spica Choir the previous year, most likely in an attempt to give her more self-esteem which she seems to be lacking in, though Hikari did show interest in singing as well. Yaya is one of the most talented members of the choir, as noted by Tsubomi Okuwaka, though seems to need more dedication, as noted by Hikari.

Source from Wikipeida