From level 2 onwards you may hire up to one Henchman in each City phase. The Henchman is used in the next non-Arena scenario you play. Use the red Ally standee to represent the Henchman.
After the scenario, read the relevant entry in the story book shown in the Hire Henchman service, depending on whether the Henchman was exhausted or not. If the Henchman was exhausted, they are removed from play and may not be hired again. If the Henchman was not exhausted, they may gain experience.