Spoiler Warning!
If you are interested in playing this game and do not want details of the final rival in the game revealed, read no further!
After you have defeated all rivals and wanderers in two out of the games three styles of contest, you receive an in-game email where the final rival calls you out.
It is '???' from the Shutokou series, driving what is termed by the game 'the Devil's Z'.
If you remember this guy from Shutokou/TXR he is described as having no known name, no stats for his car, no words spoken. Still the same in Racing Battle. If you try to talk to him in the paddock before a race you get '......'. His appearance cutscene and his racing are accompanied by a unique piece of music that is easily the most sinister of the game.
Many of the rivals, particularly the bosses, in Racing Battle have changed their cars from what they ran on the streets to something more appropriate for the track OR to whatever their sponsors want them to run. Exhaust Eve for instance does not drive her Supra, but a sponsor-covered wagon of some kind... but typically they drive a SuperGT style version of their old car. A few like Red Devil (the Char parody guy) refuse to change.
??? in every game would drive an old Nissan/Datsun Z car. He'd have it ramped up to the max, but it was basically an innocuous looking dated little blue car. Almost disappointing when you first see it, until you find out how fast it is in his hands. Well, apparently Genki finally decided he needed an upgrade in the circuit world. His new car is a 350Z in the same aero gear the car runs in SuperGT racing with the front driver's side having a ragged little corner at the front that contains the blue paint, round headlight, and fender-mount mirror of his old car. Like the new livery has swallowed up the old.

Er.. Pictures please? I'd love to see the car. Thanks for all the info!