In 2005, after seeing an audition call for new members for the Japanese idol group Morning Musume in a magazine, Kusumi considered applying after noticing she was eligible based on the minimum age requirements to participate. With her sister's help, Kusumi auditioned in secret with the song "As a Person" by Tomomi Kahara. She sang "Furusato" during the final audition and was selected for the group as the only seventh generation member, a feat which earned her the nickname "Miracle" from producer Tsunku. Kusumi debuted on the single "Iroppoi Jirettai" and was one of the founding members of Hello! Project's kickball team, Metro Rabbits H.P., in 2006.