The Free Runner subclass can be unlocked by using a Tome of Mastery after defeating the second boss (Tengu).
The _Freerunner_ can move almost twice faster, than most of the monsters. When he is running, the Freerunner is much harder to hit. For that he must be unencumbered and not starving.
Using an Armor Kit on any armor will transform it into the Free Runner Armor, which is specifically designed for this subclass.