The Voice of Reason is the smart and sensible one who speaks his/her mind about what's best for everyone. These heroes are usually the one sane member in a group of Extremists, Wrathful Heroes, Anti Heroes or just Dimwits who don't know what they're doing.
They are the ones who try their best to keep the group from getting too reckless. They are typically level-headed and want to avoid unnecessary confrontation. This may be hard for them, if they're dealing with domineering people, but they'll figure it out by talking things over smoothly.