There are 6 different Classes: Fire, Water, Plant, Thunder, Light, and Dark. Each Hero will have a fixed class and possess 3 active skills and 1 passive skill.
A Hero's skillset is not determined by its Class due to crossbreeding:
Only Genesis Heroes are bound by their Class, meaning their skills will ALWAYS have the same Class as the Heroes themselves.
An offspring’s skill class will be determined by their parents’ with the probability of 50%-50%
Because the compatibility between Classes varies, having a Normal Hero with skills that have more than 1 applied Class can give you the upper hand in a battle. Specifically:
Fire hard counters Grass
Grass hard counters Thunder
Thunder hard counters Water
Water hard counters Fire
Light hard counters Dark
Dark hard counters Light
Hard countering grants 25% more damage to the opponent.