Final Fantasy Espers

Throughout Final Fantasy XII there are thirteen Espers, or summons. After you defeat an Esper through storyline or sidequest play, you will acquire that summon on the License Board. You can then get the license with any character but only that character. In battle that character can summon that esper and the esper will fight alongside the caster until it runs out of HP or time. As your summoner gains level and ability so will your esper.

All that said they aren’t the most useful options in the world but provide some of the toughest challenges in the game. Some (Cuchulainn and Adrammelech come to mind) are available rather early in the game but will rout you until later on. Many esper fights have certain stipulations during the battle that disallow your party to attack, cast magic, use an item, etc. You will almost certainly have to do some scouting with the initial battle or inform yourself ahead of time.

Belias

The Gigas

Scion of darkness and guardian of the Holy Realm, made by the gods in opposition to Loghrif the Transcendent, scion of light. Called the Gigas for his appearance: man and monster fused as one. Considered a mistake upon his making, and receiving not his intended role, the Gigas challenged the gods and lost. Scorned by his masters, he found another: the Dynast-King, whose tomb he swore to protect for eternity.

Mateus

The Corrupt

Scion of darkness ruling and protecting those who live in the underworld, in opposition to Lahabrea the Abyssal Celebrant and scion of light. In the course of his rule, he submitted to avarice, and the darkness took his heart, transforming him until he was both evil and corrupt. Then in his cowardice did he bind a Goddess of the Demesne of Ice, and using her as a living shield, he challenged the gods. Defeated before their might, he fell screaming into the depths of hell, there to be imprisoned for eternity.

Adrammelech

The Wroth

Emperor among the scions, able to reduce to nothing aught he strikes with a single vengeful blow of his fist, created in opposition to Deudalephon the Benevolent, scion of light. Though he was made by the gods to quell the fiends that raged in the Otherworld, his immense strength and fearsome visage drew the fiends to his side, and turned him against his creators. Adrammelech rose to prominence in the Otherworld, whence he led a fiendish horde against the gods, but in the end, he was defeated.

Zalera

The Death Seraph

Heretic scion who wrapped the world in dark energies, seeking to take the souls of all living things unto himself. Created in opposition to Emet-Selch, Angel of Truth, and scion of light. Originally tasked with the judging of men upon their deaths, his soul was tainted by the curses of those who raged against the heavens, and seizing one of the gods' servants, a shamaness, as a hostage, he rebelled against his creators. Even now, in defeat, he clutches the shamaness to him in his right arm, and with the aid of her death-wail does he summon the soul of darkness to do his bidding.

Shemhazai

The Corrupt

Scion that is both horse and woman, wielding utter control over the souls that wander the underworld, in opposition to Igeyorlm the Martyr, scion of light. Though she once served the gods as a guardian, when Ultima announced her rebellion, Shemhazai went to her, whispering of the gods' hidden weaknesses. She then descended upon the land without leave of the gods, and taught men of destruction and evil. For this was she striken down and bound.

Cúchulainn

The Impure

Scion created to rid the world of its impurities by swallowing them within himself, in opposition to Nabriales the Majestic, scion of light. The world, however, was more filled with impurity and corruption than even the gods dared imagine, and having swallowed it all, the once beautiful Cúchulainn was transformed into a hideous thing, a deity of filth, and so did he turn against his creators. Wherever his feet should fall, there all life withers to dust.

Exodus

The Judge-Sal

Most ancient of the scions, created in opposition to Halmarut the Arbiter, and scion of light. Tasked with keeping watch over the world, with the authority to judge the value of all things. As he watched, unseen, unknown, his attachment to the world dwindled, and faded until it was as nothing. Fitting that he would desire to make the world, too, as nothing. Yet he fell in the war against the gods, and was thwarted, imprisoned in punishment for his heresy.

Hashmal

Bringer of Order

Scion set by the gods to wield and manipulate the laws of this world, and with holy power lead mankind to order. Created in opposition to Fandaniel the Protector, scion of light. Desiring to bring order to all things, he joined with Ultima in her battle against the gods. He gave his body to the Thousand-Years War, and when his strength was spent, down into the burning inferno he fell.

Zeromus

The Condemner

Honoring the law more than any other, a scion of holy order and condemner of criminals. Created in opposition to Knight-Star Pashtarot, scion of light. He turns his deep, abiding hatred for those who break the law into living darkness, therein to plunge the guilty in fell judgment. Over time, he came to care less for upholding the law and more for condemnation, and so tainted by hate, he sought to condemn the gods themselves to death. Thus, did he earn the title "The Condemner", and thus did he fall from grace.

Famfrit

The Darkening Cloud

The hideous, darkly clouded form of Famfrit, scion in opposition to Holy Queen Emmerololth, scion of light, was anathema even to his creators. Thus, after a great battle, was he broken and sealed within armor laced with wards. The confines of his armor are void of light, and so is he called the Darkening Cloud. Men fear the rain that falls from the black clouds that ooze from that giant ewer as a herald of chaos and waste.

Chaos

Walker of the Wheel

Tutelary deity of the sacred crystals fashioned by the gods at the time of the Great Making. Created in opposition to Mitron the Chasiter, scion of light. Upon entering the world of Man, he was enveloped in the turmoil rampant there. Lost, he died and was reborn countless times, a walker of life's wheel, eventually to rage against the gods that had so failed him. By sitting in meditation upon the Unrh Pedestal does he clear heart and mind until all that has order and reason and thought is made as nothing.

Ultima

The High Seraph

Masterpiece among the scions created by the gods, and the mastermind of the plot to rise against them. Prior to her betrayal, she was tasked with guiding souls to heaven and aiding in their reincarnation. Called the High Seraph for her angelic wings of glimmering gold, yet it was on wings of deepest black that the tainted angel Ultima rose against the gods. Since her fall, her heart is without light, and impossible to know.

Zodiark

Keeper of Precepts

Strongest of the scions created by the gods, they feared his growth, and so kept him a child. So indomitable is his strength that all things are by him twisted and pressed into oblivion. He alone fashions the laws governing all things, and administers punishment in place of the gods. So is he Keeper of Precepts, and his authority is absolute.