[Black★Rock Shooter] A Preview in July Issue of Hobby Japan

It was revealed a third human character. The magazine is referring to her only as “Yuu” (ユウ), and has revealed her voice actress as well.

Yuu appears to be the hyperactive loli-ish type, and there’s no confirmation on whether she’s just a side character, or directly involved in the story.

Yuu may be played by Asumi Kana (阿澄佳奈), the same seiyu who gave voice to Ran (Shugo Chara & Shugo Chara!! Doki—), the female attendant in episode 11 (Rosario + Vampire), Megumi Kayano (Persona -trinity soul-), Sumae Mizuno (Rosario + Vampire Capu2), Sakura Kano (Toradora!), Irie (Angel Beats!), and many more.