A hero with leadership skill, and has activation requirement: effective when stationed in the barracks..

Being stationed in the Barracks means to be added to an APC to lead troops. You have to do research in each of the 4 APC research trees (when you have them unlocked) to allow you to start adding heroes. All combat heroes can be used when you have space in the APC to lead troops and the leadership skill will let you add more troops making the formation stronger.