Aki Toyosaki
Kotobuki performed the opening and ending themes of the 2009 anime series "K-On!", alongside Aki Toyosaki, Yōko Hikasa, Satomi Satō and Ayana Taketatsu. She performed the opening theme for "Hatsukoi Limited" series, with Aki Toyosaki, Ayahi Takagaki and Haruka Tomatsu. The four are affiliated with Music Ray'n, an artist management and publishing group of Sony Music Entertainment. They are currently known as the musical group Sphere and released their first single, "Future Stream", on April 22, 2009.