Final Fantasy VI
Espers in Final Fantasy VI were initially created during the War of the Magi, after humans came into contact with both Magic and the Warring Triad. When the goddesses realised that they were doing wrong, they decided to seal themselves away and used the Espers to protect them.
The Espers lived peacefully for 1,000 years, untl a girl named Madeline managed to stumble into the Esper world. She fell in love with and Esper called Maduin and together they had a baby girl called Terra Branford.
Shortly after Terra's birth, the Imperial Army lead by Gestahl also stumbled across the Esper world. They weren't peaceful though, and captured many Espers including Maduin and Terra. However, before too much damage could be done, the Espers manage to form the Sealed Gate, to protect their world.
The Imperial capital of Vector served as a prison for all of the captured Espers, and this is where they were experimented on by Dr. Cid. He managed to extract their power and infuse them into a human, with the first test subject being Kefka.
If an Esper had its power completely drained, it would be reduced to a Magicite. Eventually the majority of the captured Espers ended up as Magicites, which fell into the hands of Kefka.
Harnessing Their PowerThroughout the game, many summons can be used by players. Once they are turned into a Magicite, their power can be harnessed and because of this, they can be used to teach various spells.
As well as allowing magic to be learnt, the Espers can also be summoned during combat to provide assistance, whether it be from a defensive or offensive perspective.
Final Fantasy Tactics
The Espers present in The War of the Lions are unknown in origin, but one of the main villains is said to be of the same origin as the Espers found in Final Fantasy XII. They are used in a very similar way to many Summons, in that they are called during a battle to perform a singular action.
Final Fantasy XII
The Espers featured in Final Fantasy XII were originally created as Scions by the Gods, but it's unknown what purpose they served. Each of the Espers featured in the game has a Zodiac sign associated and there are thirteen Espers featured in the game.
While the Scions were created by the Gods, the highest ranking Esper, Ultima, rose up against her creators. She allied with the other Scions of Darkness and attacked the Gods, but they were all defeated and banished to Ivalice. Zodiark, the thirteenth scion was also banished because the Gods feared the power that he possessed.
Fusing Espers To One Character
Due to the licence board system, it meant that Espers could only be associated with one specific character. However, each character could have three Espers bound to them.
When an Esper was summoned, the rest of the party members disappeared and only the summoner and Esper remained on the battlefield. This is similar to the mechanic introduced in Final Fantasy X. They acted as a new member of the party, with their own list of gambits and special moves that could be performed if certain conditions were met.
Final Fantasy XII: Revenant Wings
Although many people call them Espers in Final Fantasy XII: Revenant Wings, they are actually called Yarhi. While the Scions from Final Fantasy XII do exist in Revenant Wings, there are also numerous Yarhi which are completely independent and are known as the Espers of Lemures.
These were created from a stolen anima of the Aegyl, through the use of an Auracite. The stolen Aegyl souls gathered and managed to create the World of Ollusion, where they called the material world via a Summoning Gate.
The Ring of Pacts
Espers in Revenant Wings are controlled through the Ring of Pacts. This is where Espers can be bought, and they are catergorised into six main areas. Each of these elements has three ranks, which can determine the strength of the Espers.
While the typical elements are present, each Esper is also categorised as either Melee, Ranged or Flying. They are all strong against another type, in a triangular weakness formation.
Final Fantasy Tactics A2: Grimoire of the Rift
Summoners have a monopoly over Espers in Grimoire of the Rift, as they are the only class that can actually summon them in battle.
Esper is a fairly liberal term in this game, as there are many different types of Summon. Each of them are still referred to as an Esper, and none of which have any particular back story.