Prior to debuting professionally, Kōki had written three songs for her mother Shizuka Kudo's album, Rin, in 2017, as well as for Mika Nakashima. In May 2018, Koki debuted as a model, appearing in the July 2018 issue of Elle Japon. Kōki has also appeared on covers for fashion magazines such as Numéro Tokyo, Grazia China, Nylon Japan, and Vivi. In August 2018, she became Japan's youngest ambassador for the luxury brand Bulgari. In October 2018, Kōki appeared in her first commercial, endorsing products from Otsuka Pharmaceutical.
