This is actually not obvious. As the roller is cleaned at 90°C, I'm pretty sure it's microfibre. No bristles, more like fabric. Gentle on wooden floors. We only have real wood oak parquet (open-pored, not varnished) and only use this appliance to wet clean the wooden floor with WOCA special soap. This has worked for several years without any problems for the wood.
It's made from heat-resistant synthetic fibres, specially designed to absorb water and withstand high-temperature cleaning. This is confirmed by Dreame's official information, which speaks of "heat-resistant brush fibres" and "high-quality materials".