@Zeffy: It's the Shenshou-Jing control unit that was interfaced with Miku's brain, and which was responsible for turning her ordinary love for Hibiki into an insane burst of violence that nearly killed them all.
My fragile heart... Miki x Seina is love, Miki x Seina is life! Why you tow are so much cute?!
By the way, that Sasa is so cool...
...and Aki so much rock! A real Rock Goddess! (Aki for president!)