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. They are currently known as the musical group
Sphere and released their first single, "Future Stream", on April 22, 2009. Coupled with the release of her first
album My Stride, Kotobuki had her first solo Japan-wide
concert tour entitled
Our Stride in 2012 spanning across four dates and venues starting from November 18 in
Kobe's International House Kokusai Hall and ending on December 9 in
Ōmiya's Sonic City Hall. Kotobuki is said to be "the fastest learner on how to dance" among her peers as far as her being a member of Sphere goes so much that she even guides the other three members of Sphere on dancing. Even in singing, Kotobuki is said to learn just as fast, "a pro in harmonization" according to her fellow castmate in
K-On!,
Satomi Satō, and "someone who has good sense in pitch" according to one of
K-Ons staff members in charge of music. During her stint in both seasons for
K-On!, she and her fellow cast members were involved in two live events, and just like the character she portrayed in
K-On!, she was in charge of playing the
keyboard in both occasions. She also played the keyboard on some of her own songs during her
Our Stride solo concert tour and on Sphere's first 2013 live event,
Music Rainbow 02. She also participated in the anime series
Aikatsu! and was the voice actress of Kanzaki Mizuki. == Personal life ==