Koike started a junior idol career with the DVD Big Brother at age 10, when she was in 4th grade. Koike Rina played Sailor Luna in the 2004 live-action version of Sailor Moon, Pretty Guardian Sailor Moon. She also played the role of Shizuka on the 2008 Kamen Rider Series, Kamen Rider Kiva. In early 2011 she starred in the short TV drama Onegai Kanaete Versailles, alongside visual kei metal band Versailles.

After graduating from Asia University in 2016, Koike took a two-year break from the entertainment industry to spend time studying English in New Zealand and Australia.