FFXII - Espers
As with every Final Fantasy since Final Fantasy III, summons return in Final Fantasy XII in the form of Espers. Beings of darkness created to counter beings of light, they fell from grace and are imprisoned throughout Ivalice.
They total 13 in all - 1 for each Zodiac symbol. Many are associated with one of the 8 elements in FFXII (and use the corresponding "-ja" spell as their major attack) and several contain stipulations over the course of the battle (i.e. magic command disabled, attack command disabled, etc.)
Each esper has an item that can be stolen with regular, medium, or great difficulty. Items can range from the ridiculous (Pebble) to glorious (High Arcana); the one thing each esper has in common is that the gem corresponding to their zodiac symbol is one of their stealable items (for example, Belias' zodiac sign is Aries; the Aries gem can be stolen from him. For Zodiark, the stealable item is Serpentarius, the latin translation of the 13th star sign "Ophiuchus.") Only one item can be stolen, though (potentially more than one with Thief Cuffs equipped) so you're not permitted to steal multiple times as in some past Final Fantasies.
Five espers are fought as part of the storyline; the other eight are optional. The optional ones can be encountered entirely by accident, or via a lengthy sidequest. All but Shemhazai and Zodiark are based on major villains from previous FF's: specifically, the final bosses of the first five Final Fantasies, and the Lucavi demons from Final Fantasy Tactics.
Each page details the respective esper's bio, how to locate/obtain them, and battle strategies (Note: the attack damage figures given are an approximation, and fluctuate depending on level, equipment and character stats.)
Adrammelech
Belias
Chaos
Cúchulainn
Exodus
Famfrit
Hashmal
Mateus
Shemhazai
Ultima
Zalera
Zeromus
Zodiark