5. Daihatsu Copen
Daihatsu is one of the prime manufacturers of Kei Cars with its entire range dedicated to those small vehicles. Their most exciting model is definitely the Copen, a tiny roadster. If you want to visualize what Copen is like, try Miata but the half size.
The Copen is introduced in the early 2000s and is in its second generation. As expected, the 660 cc engine is under the hood, and power is below 70 HP mark. However, the turbocharged engine is highly tunable, and if you want, you can get some pretty lively performance from this compact roadster.