The icon in the upper left corner of the hero's avatar indicates which faction the hero belongs to.  

During the battle, faction suppression may occur and damage caused to the suppressed opponent will increase. Faction suppression information can be found in the battle formation interface by clicking the "!" icon. If your heroes meet the conditions of a faction combination, additional buff may be granted. Tap the two icons in the upper left and upper right corners of the battle formation interface to view your buffs of those of your opponent respectively.