Not having read the actual work, it's pure speculation. But the main characters could be the riders, who might be the protagonists trying to navigate the world shaped by the wage book. There could be antagonists too, maybe those who are trying to maintain the status quo of the wage book system for their own gain. Additionally, there might be characters who are caught in the middle, ordinary people whose lives are affected by the riders' actions and the wage book system.
Since I don't know the specific details of this 'riders of the wage book science - fiction', I can't say for sure who the main characters are. But likely they would include the riders themselves, perhaps some authority figures associated with the wage book system.