Who is the singer of "Future Star"?
Yes, it's true. The voice actors (Risa Taneda, Minori Chihara, and Yuri Yamaoka) themselves sing "Future Star". I mean, the voice even matches in the song and the voice acting of anime.

If you're still skeptical, refer the source below:


VGMdb - Beyond the Melodies
Wikia - 
Kyoukai no Kanata Original Soundtrack "Beyond the Melodies"
Wikia - Yakusoku no Kizuna


Also, I am pretty sure voice actors weren't the one dancing in the live show concert. They were some cosplay artists.