Keiichiro Aizawa (Also known as "Kei" in the games) is the character who drives the Supra RZ/Supra Twin Turbo, dubbed as the "Monster Machine" (???????? Monsuta Mashin?) tuned by Kou Tominaga, Kazuo Ota, Kazuhiko Yamamoto and Akio Asakura. After meeting Blackbird, Kei decided to chase after him.

His desire for racing and his car choice was inspired by his father, Koichi Aizawa, who drove a Mk.II Celica Supra and was killed by a drunk driver 15 years before the anime is set.

He works in a host club to finance his racing activities, and he has friends from high school who urge him to leave the dangerous high-speed Wangan racing behind.

After he lost the race against the Devil Z and Blackbird due to structural instability, Kei quit his job and agreed to let the tuners detune his 800HP engine to 400HP. The tuners also repainted his Supra to the same Silver as his father's.