Ryoma is a hard-boiled dandy character. Though he looks like a child, it's been said that his life experience would put an adult to shame. He has a coolness that common high school students don't have, heightened by his very deep, mature voice. He is the calmest, most composed, and an unperturbed person in the Killing Game. 

Here are some voice examples: https://youtu.be/PhyacTkJhvU