Seeins as my Phoenix Down thread was interesting, I am going to give my theory on the wonderful and amazing allied monsters with cool intros.

First let me start with Summons.....

I theorize that a summon is triggered by one of a few things or a combo of them. First is a chant similar to a spell. Second is a dance which would call upon the creature. Third an inner power released by the will of the summoner. Four a mental trigger link to the beast. Example, Cloud calling to Bahamut in FF7 (any Bahamut form will do Neo/Zero/regular)

Next I would like to give my theory on Espers......

The creatures called upon in FF6 are referred to as Espers. I figure the Esper is actually a part of the soul of the person that releases them. They are usually found through a magic item, in the case of FF6 magicite. I would assume that the user is born with this power and it lies dormant until the user figures out they have this gift. OR that it is released during a time of extreme distress, danger, sorrow, sadness or any emotion that heavily impacts the person. Example: Terra calling upon Shiva or Ramuh in FF6.

Third is more of a supplement to the Espers, but they are wearable....Relics.

Relics in the form of armor, attire, even an item (ex: Moon Stone, Ancient Tablet). A relic is something that holds the ancient creature within it. Relics wait to be found and used, I would assume that they most likely will not just answer to any person who stumbles upon it. One would need to harness the power of the relic, most likely through a proving trial or something to that effect. I cant give an example because this is just a supplement.

Fourth is the Aeon.

Aeons made famous in FFX. Only Yuna (in your party) can call upon these majestic personal bodyguards. Aeons are embedded in the summoner and must be learned. Similar to a summon there may be a chant or a dance/movement involved in calling upon them. Aeons are 100% truly loyal to the summoner because they are in part an extension of the person. Example is Yuna calling Valefor, when he comes to her side she pets his head and caresses his face lovingly because he is part of her.

Finally my 5th theory is the Eidolon

See FF13, FF9, FF4. Eidolons have to be learned through some HIGHLY personal trial to the chosen user. According to FF13 which is what I am basing this theory on, Eidolons are specific to the person. Example, Hope overcoming Alexander and being able to call upon the monstrous flying cannon fortress.

Ok its 4:18AM EST, im tired. Opinions on my theorys please.