Espers are Final Fantasy XII's versions of summons. These powerful creatures can be called into battle to aid in the fighting. They will take the place of two party members, who did not summon it, for a 1:30 minutes or the Esper and summoner are defeated.
Each individual Esper has their own skills, Gambits, and Ultimate attacks. Almost all of the Espers have an elemental weakness as well, making them less useful in certain battles than others.
You must unlock the Espers on the License Board, before you can summon them. You also have to find and defeat them, before they will appear on the board. And you thought you were going to get them easily? Five of the thirteen Espers will come from the main story, while the others require a bit of side questing. Though you might not be keen on side quests, you will definitely want to get a hold of these beasts.
Once defeated, you then can unlock the license for them. Once a character activates a Esper panel, the panel will no longer be available. Like the Myst charges these are one time only panels. Try to spread the Espers throughout your entire party, or at least through your three main fighters. This way you can take full advantage of each Espers abilities.
Summoning an Esper also requires Myst charges. Espers will take one, two or all three Myst bars, depending on the power of the Esper. A Esper such as Belias who's power is reletively weak only takes one bar, where as Ultima takes the full three bars to summon.
It should also be noted that in XII, Silence no longer affects the ability to summon. The only thing that would prevent you from summoning is if you do not have enough Myst charges.
Espers are treated like the visiting characters in the game. As stated before, they have their own gambits and attacks. You will not be able to change leaders while the Esper is on the map. Without the person who summoned it, the Esper could not be there right?
The level of the Esper is determined by the summoner who is equipped with it. This will determine the Espers stats. So the higher the summoner is in levels, the higher the Esper will be.
Once the Esper has been summoned, your main fighter will continue with their normal fighting via their gambits. This is ok, but your focus should be keeping the Esper alive throughout its fight. Though with most Espers, you will not have to worry about status effects, since thay are immune to them. Some will also come equipped with effects such as Faith, Bravery, Reflect, Shell, Haste, Libra, and Protect. If you have an Esper with the Reflect on, make sure to quickly equip a Opal Ring to negate the reflection. This way you can effectively heal your Esper. If your Esper has no stat boosts such as Haste, Protect, or Shell, you should consider casting it on them. This will give your summon more time on the field, and makes for a better chance of getting their final attack.
In here you will find each Esper, and their vital stats, gambits, and attacks. You can also find the LP charge needed to unlock them, and their stat bonuses. Try to get all thirteen of these awesome beasts. you will be greatful in the end if you do.