A Hero Card shows all of a Hero's stats.
Element - Dr Vortex, shown here, has a dark (purple) element. This is shown both in the top left corner of his Hero Card, and also by his general colouring.
Hero Class - A Hero's class (Mage, Battlecaster etc) gives an indication as to how their stats will be balanced.
Power - the star rating indicates how powerful the Hero is. Generally, four-star Heroes can be evolved into 5- and then 6-star ones by levelling them up with fusion.
Level - the Hero's level relative to their max.level.
Evolves At - once the Hero reaches this level, it can be evolved.
Padlock - tap to toggle whether a Hero is locked for fusion.
Hit Points (HP) - when a Hero is attacked, they will lose HP. If HP reaches zero, that Hero will die (although Heroes never fully die, they are just removed from the current quest)
Attack (ATK) - The higher this is, the more powerful the Hero's attack, and the more damage they'll inflict.
Defence (DEF) - The higher this is, the less damage a Hero will take when attacked.
Recovery (REC) - The amount of HP a Hero recovers each turn they are on the backline (ie not at the front of the Revolver).
Special Move (Siphon shown)- Heroes can use their Special Move when they have collected enough Mana in combat.