Career
Koike made her acting debut in the Japanese television drama Botan to Bara as Rika Nojima in 2004. In 2007, Koike appeared in Nippon Hōsō Kyōkai Educational television"s Yuhigaoka Boy Detectives (夕陽ヶ丘の探偵団, Yūhi ga Oka no Tanteidan) as Hikari Tsukishima, the only female in the detective group. 2007 was also the year Koike made her first appearance in films starring in Sakuran and Always Zoku Sanchōme no Yūhi.
In Sakuran, Koike played the part of the young Kiyoha, the main character of the story.
In Always Zoku Sanchōme no Yūhi, Koike played as Mika Suzuki, a relative that moves in to live with the Suzuki family at the beginning of the movie after her father"s business ended in failure. In 2010, Koike played Mizuki Tsuji in the stage production Alice in Deadly School.
In 2011, Koike was starring in Into the White Night, the theatrical adaptation of Journey Under the Midnight Sun. Koike and the other two receivers of the award, Kenta Suga and Kazuki Koshimizu, were granted the award in recognition of their performance in Always Zoku Sanchōme no Yūhi.
Koike released her first photobook on February 13, 2009.
Koike commented on one photo where she holds a scarf up to her mouth, stating that she liked it because the look of her gaze staring straight at the camera made her look like an adult. Her mother was reported as being quite shocked about how mature Koike looked in the photographs. Koike"s first idol Digital Video Disc named Hatsukoi (初恋) was released in 2009 and her second Digital Video Disc named Soshite, Chiisana Koi (そして、小さな恋) was released in 2010.
Koike is one of eleven girls in the girl group Tokyo Cheer Cheer Party.