Black Rock Shooter

Mato Kuro, the main character, comes to high school and met with Yomi Takanasi. Classmates have become best friends and spend all their free time together. But a year later their "throws" for different classes, and now devotes her friend Mato less attention. It introduces a new girlfriend - Yuu. Once Mato know that Yomi has disappeared and is donated by a friend to phone pendant in the form of blue stars - the exact same as her own.
As soon as she regards the suspension, she immediately falls into an alternate world where it is - the opposite of itself. From now on it - Black Rock Shooter, created in order to kill. Its aim - to find and destroy at least a mysterious girl named Dead Master, which is actually missing Yomi. (Source: gLaKs)