Eidolans are your summoned fighters. Your main hero does not fight, they are part of the storyline but not the combat.
You can gain them from events, summoning, the temple, VIP and recharging.
There is 1-5 star eidolans but not all are used in combat, some like the demonic onions are devoured for soul which is used to evolve the eidolans.
Eidolans can be leveled up to each 10th level then they need to be evolved, using items from in game events
Once evolving is done, the eidolan can be leveled up another 10 levels using soul which can be gained from events and devouring demonic onions
The items for evolving the eidolans can be gained from events like gauntlet and campaign, as each new level and evolve state is completed, the next evolve requirements will increase for items and amounts