Astels are as important as Astellia Asper in Astellia Online. 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, as you improve your Astel Star Ranks, having certain Astels in your deck will give you Deck Buffs.
You can register Astels in your Deck Slots. This is represented by a hotbar at the bottom of your UI by default. By using the F1-F8 keys, you can summon these Astels. It costs a certain amount of AP when summoning Astels, which you can see in the Astel Encyclopedia. Your AP will drain quickly if you have more than one Astel summoned, 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. 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.
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. You will gain SP from looting cards once you have an Astel. SP is how your Astels gain Star Ranks, which will not only allow you to slot higher grade Star Jewels in your Astel, but increase their power.
Star Jewels have bonus stats to attack or defense, 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 by looting from monsters, or via Archaeology.
(An Astel can't use equipment like characters do, but they can be equipped with Star Jewels that can boost their stats.)
When you gain EXP, you can 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 will start at level 1 automatically.
Depending on the Astels in your deck, you can gain Deck buffs. The deck buff that 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.
(If the total points of the Astels added to the deck exceed a certain number, an Astel Deck Buff is activated.)