Each of the 60 plus characters in Skyland Ring of Heroes belongs to one of 10 elements (earth, air, fire, water, dark, light, undead, life, magic, and tech) and is strong, weak against another element. If the element of the enemy you are fighting is weak then your strong element character will deal extra damage in the battle.
As well as belonging to an element all the characters specialise in one of four classes (support, defense, attack, and expert). You can always check the element, class and stats of a particular hero by going to the 'Skylanders' menu and tapping on the character. The class of the characters breaks down as follows:
Attack Type Characters
These heroes provide ATK boost in the battle (passive skill, active skill).
Defense Type Characters
These heroes provide DEF boost.
Support Type Characters
These heroes support allies during the battle.
Expert Type Characters
These heroes specialise in certain skills (low cooldown).
To unlock a character you first need to obtain the required number of soul stones or fragments. Once you have them go to the 'Character' menu, select the Skylander, and press the 'summon' button. You can initally check to see how many soul stones are needed to unlock a hero by tapping the 'stats' tab of the Skylander in question. It is also here if you tap the 'add' button that you can see a list of stages where you get the selected character's soul stones/shards/fragments.