Millie, known as Rie Katori in the Japanese version, is a mechanic for the Katori Motors team. She operates from the country retreat where the protagonist recovers and advances his racing career. Millie possesses considerable mechanical aptitude and technical skill, directly contributing to vehicle development and maintenance. Her role expands beyond technical support to include operational assistance during team activities. She exhibits unwavering loyalty and provides consistent emotional support to the protagonist, especially during professional setbacks or personal discouragement, functioning as a stabilizing element within his support structure. Millie regularly interacts with other team members, such as Joji and Sumiko Kurama, though the precise nature of these relationships is not detailed. Her personal background and history are not explicitly documented in official media. The character maintains her established supportive function without significant evolution or altered role throughout the original 44-episode series narrative, with no verified appearances or expanded development in spin-offs, films, or supplemental media.