Sayuki (沙雪) is Mako's co-driver/navigator.

Sayuki is a cheerful, outgoing girl and she is always looking for good looking guys like the Takahashi brothers. She also developed a crush on Takumi, finding him cute and invited him to a pool after his race against Mako and Sayuki. As navigator, she gives out instructions to Mako when they're racing (when to brake, what drifting technique to use etc.). Her last name is unknown.

(Source: Initial D Wiki)