Astels are an important part of your gameplay in Astellia. Not only do they help you kill things faster, but they can provide beneficial buffs, combination attacks, tank, and heal, depending on the Astels you choose. Furthermore, having certain Astels in your deck will give you Deck Buffs, as you improve your Astel Star Ranks.
You have 8 Deck Slots, where you can register Astels. This is represented by a hotbar at the bottom of your UI by default. You can summon these Astels by using the F1-F8 keys. Summoning Astels costs a certain amount of AP, which you can see in the Astel Encyclopedia (J by default). If you have more than one Astel summoned, your AP drains quickly, and once it reaches zero, the last Astel you summoned will be automatically dismissed. The only exception to this are Savior Astels. Once they have been summoned, they do not drain AP, but have a limited time on the battlefield before they are automatically dismissed.
Astels have three types of skills. The first Type are their normal, default skills, which they will use automatically. The second set of skills are combination skills, which they will perform based on a specific skill you use. This can be seen on the Astel page in the Astel Encyclopedia. The last type are forced use. To activate them, use the F1-F8 keys, while the Astel is summoned. Using these skills costs a specific amount of AP. Summoning Astels costs a certain amount of AP, which you can see in the Astel Encyclopedia (J by default). If you have more than one Astel summoned, your AP drains quickly, and once it reaches zero, the last Astel you summoned will be automatically dismissed.
In order to summon more Astels, you can use Atra Tonics, which can be gained as loot from dungeons, or by purchasing them from the General Merchant. This will allow you to have two Servant Astels out at the same time.
There are two ways to gain new Astels. The first is to complete the main and side quests. The second is to do dungeons and collect their cards as loot rewards. Once you have an Astel, you will gain SP from looting cards. SP is how your Astels gain Star Ranks, which will not only increase their power, but allow you to slot higher grade Star Jewels in your Astel.
Star Jewels have bonus stats to defense or attack, and are an important part of improving your Astel's abilities. You can slot them in your Astel via the Star Jewel button on the Astel page, which you can access in the Astel Encyclopedia. Star Jewels can be crafted via Archaeology, or looted from monsters.
As you gain EXP, you share it with the Astels currently slotted in your deck. Astels level much as your character does, but cannot outlevel you. If you gain a new Astel, it automatically starts at level 1.
Depending on the Astels in your deck, you can gain Deck buffs. The deck buff you can gain is listed on the Deck Effect tab of the Astel page, as well as the Astels required for that buff. It takes certain star ranks to activate and improve these deck buffs, which you can also see on that page. Once you have successfully met the requirements for the Deck buff, an icon indicating it will appear next to your mini map, in the top right hand corner of your screen.