Riku was born human, just like Kai.
Again, you need to go back to the episodes in Vietnam when Riku was captured. It seemed pretty evident that all the children he were with had been used in some expirement turning them into Chiroptera. They were all humming Diva's song and obeying her commands.

Riku had just not been through enough experiments to turn into a complete Chiroptera, but it isn't like he was born that way or anything. Some people have the memory of a fish...