When you start the game, you'll be introduced to the main character that you'll play the story mode with - the 7th in line heir to the throne of Alberian Kingdom. As you progress through the chapters of the game however, you'll be able to earn more Characters, as well as be able to Summon your own roster of adventurers later on.
Once you get past the tutorial phase of the game, you are free to Summon Characters as long as you have Wyrmites or Diamantium. By default, you get to spin one random character, dragon, or wyrmprint (usually for 100 Wyrmite per summon). However, this is not the only option when you get to the actual summon section of the game. In fact there are 3-4 options to choose from when summoning: via Free Vouchers (randomly given by the game), Daily Deal (where you basically pay via in-app purchase), Single Summon, and Tenfold Summon (which guarantees at least one 4 or 5 star item).
Characters are also sub divided by classes (generally indicated by their choice of weapon) - which gives you the variety of options to put together in one party. Apart from that, each of them have also their own elemental affinity - which dictates how effective they are against certain types of enemies. In the game, there are 5 main color types; green - wind element , red - fire element, blue - water element, yellow - light element, and violet - shadow element. The basic setup of character effectiveness or ineffectiveness is as shown on the image below.
Aside from that, you also have 3 different character rarities in the game including Basic Adventurers (3-star rating), Rare Adventurers (4-star rating), and Legendary Adventurers (5-star rating). The same principle is applied to dragon summons (and their rating) in the game. 3 Star Adventurers and Dragons - are the most common Characters that you'll most likely get while summoning for additional characters. 4-star adventurers and dragons on the other hand are rare, but the chance to pull one out is guaranteed when performing 10-fold Summon. The rarest characters (5-star) are the most powerful characters that you can get in the game, but are very hard to obtain.
Leveling Up your Characters
To level up your Characters, you must have them enlisted to your roster of characters. You can enlist a maximum of 4 characters in your roster. By default, your adventurers have a ceiling level of upgrade. A higher cap level can be unlocked by improving their Mana Circle. When you successfully unlock all the Skills in one circle, you’ll get the chance to unlock a higher tier. Each mana circle unlocks a special skill for your character, and unlocking skills in higher tiers, means you’ll get better performing adventurers. Also, each upgrade will give your character a permanent boost on a particular stat, and his/her overall battle rating. Take note though that you'll need mana (usually earned from doing Quests) to actually perform these upgrades to your character as well as a certain amount of special items -- when unlocking a higher tier of the mana circle.
The best course of action to have your roster upgraded steadily is by simply playing through the chapters of the game until you run out of energy. Make sure that you also check the Endeavors and try to accomplish their requirements -- for they usually give you additional reward items in check on return.
Additionally, check on you Halidom and try to progressively improve the setup of your facilities. Knowing what elemental types of Characters you usually use -- will generally help you figure out which facilities you might want to build and upgrade in order to boost your adventurers.