Here are a few interesting Kei cars that aren't talked about enough. None of these made an appearance in GT.
1972-1978 Subaru Rex GSR:

The GSR was the sportiest model of the first-generation Rex. Featuring a fine-tuned suspension and sports tires, it was a star at taking tight turns. It's tiny two-cylinder engine produced 37 horsepower at most, which doesn't seem like much at all. However, this car was much lighter than it's competition, weighing in at under 500kg.
2004-2009 Daihatsu Trevis GT:

The GT was the sportiest and most upmarket trim of the uniquely-designed Daihatsu Trevis. Visually, it was a smaller, but more modern rendition of the original Mini Cooper. It's engine was not supercharged, but still produced 63 horsepower, which was powerful by lei-car standards. The Trevis GT was never sold in Japan (sold only in Europe).
1981 Suzuki CV1:

CV1 was perhaps Suzuki's most daring attempt at creating a new kei car, besides the Cappuccino. "CV" stood for Community Vehicle, and it was supposed to be "the kei vehicle of the future". Powered by a 50cc engine, it was not fast at all, topping out at 21 mph. It was very light, though, weighing 160kg. Unfortunately, the CV1 never really caught on with the Japanese public, therefore only a small batch of these were sold.