竜崎天音（または藤井美鈴）さんについて

A friend I became close to through our shared love of our favorite idols was murdered. She was a very important friend, someone I could confide in about anything... Until I learned that her name, address, everything about her was a lie. Who was the murdered friend, Fujii Misuzu (a pseudonym)? Why did she have to be killed? As Kiryu Kokoro is consumed by suspicion, he receives a direct message. The sender is... (Source: Kadokawa, translated)