|First appearance||Episode 1: Dissolution!|
|Voiced by||Yōko Hikasa|
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Mio is a member of the light music club. She originally intended to join the literary club, but is forced into the light music club by her childhood friend and the club's drummer, Ritsu Tainaka. She gets excellent grades in school, but is bad with listening to stories involving macabre experiences, and cowers whenever something gross is brought up. While she can be mature and strict sometimes, she gets embarrassed easily, and is often subject to teasing from Ritsu and Sawako, their club adviser. She cites that she chose bass since it is not the center of attention in the band, unlike the guitarist. Mio is more technical when it comes to music, and Yui often comes to her when she is in need of more guitar tutorials. She writes the club's songs, although they feature some odd lyrics like. As she is left-handed, she becomes entranced whenever she sees left-handed instruments. Due to an unexpected accident in one of their live performances, she acquired a huge amount of popularity, even a fan club for her is formed (much to her dismay considering the circumstances surrounding it), whose founding member is the former student council president, who also stalks Mio. Because of this, most of her classmates voted her to play as Romeo in their class play.
Her surname is taken from former P-Model bassist and keyboardist Katsuhiko Akiyama.