As in Final Fantasy XII, enemies are integrated into the open field and can be approached or avoided by the player. When the player's character touches an enemy, the screen transitions from the regular map to a separate battle screen similar to those used in previous Final Fantasy titles.A maximum of three characters may be used in battles, which use a variant on the series' traditional Active Time Battle (ATB) system designed by Hiroyuki Ito and first featured in Final Fantasy IV. Under this system, the player selects an action from the menus, such as Attack, Magic, and Item. Unlike previous games in the series, the player only controls the lead character while the remaining two characters are controlled by the game's artificial intelligence (AI). Each action requires a specific number of slots on the ATB bar, which continually refills to a set maximum number of slots. The ATB bar gradually increases in size throughout the game from two slots to six.The player may select less than the maximum number of possible actions or may stop the filling of the ATB bar and perform as many actions as can be done with the current ATB amount. The player may select an autobattle command, which fills the ATB slots with actions chosen automatically. Actions cannot be performed outside of battle, and the characters' health is fully restored after each battle.
Each enemy has a meter, called a chain counter, consisting of a percentage starting at 100 which increases when the enemy is struck by attacks or spells. Attacks by different roles have different effects; some raise the chain by a larger amount while others give the player longer before the chain counter resets. The amount of damage performed by an attack is multiplied by the chain percentage before it is applied to the enemy. When the chain counter reaches a preset amount, different for each enemy, the enemy is placed into Stagger State. In this mode, the enemy has lowered defense and may be launched into the air.The Paradigm system allows the player to program six different roles which the characters can then assume to perform certain formations in battle in response to the specific conditions. The roles consist of Commando, a warrior-type role; Ravager, a black mage-type role which uses damage-dealing magic; Medic, a White Mage-type role which can heal; Saboteur, which can weaken enemies; Synergist, which can strengthens allies; and Sentinel, which has protective abilities. Each of the characters can initially take on only three roles, but the player has access to all of them later in the game (although the other three roles are limited in their abilities for those players which choose them). The player can select which roles the controlled character and the AI characters are using both outside and during battle, which is the only way that the player can control the AI characters during battle. The player can only choose from specific sets of paradigms that the player has set up beforehand outside of battle.
Each character can summon a specific Eidolon into battle.These summoned creatures include series staples Odin, Shiva, Alexander, and Bahamut, and newcomers Hecatoncheir and Brynhildr.When summoned, the Eidolon stays in combat while the characters accompanying the summoner leave the party. While an Eidolon is summoned, the player can trigger a feature called Gestalt Mode, in which the Eidolon transforms into a different form and performs different attacks while the summoning character rides them.

Crystarium

The Crystarium is a leveling system consisting of six crystals and resembles the Sphere Grid from Final Fantasy X.Each crystal in the Crystarium represents one of the six Paradigms, and is divided into ten levels. Each level contains various nodes that supply bonuses to health, strength, or magic, or provide new abilities and accessory slots. These nodes are connected by a semi-linear path. The player may advance down the path by acquiring Crystarium Points, which are awarded after defeating enemies. The full Crystarium is not available to the player at the beginning of the game; at specific points in the game's plot, the player gains access to new crystals or levels

Characters

The six main playable characters of Final Fantasy XIII are Lightning, the main protagonist of the game, a former soldier and older sister to Serah; Snow Villiers, Serah's fiancee and leader of NORA, a paramilitary group;Oerba Dia Vanille, the game's narrator and an exile who is later revealed to be a l'Cie from Pulse; Sazh Katzroy, a civilian pilot and father to a young boy, Dajh;Hope Estheim, a young boy who is struggling within the relationships he shares with his parents;and Oerba Yun Fang, a l'Cie from Pulse who is working with the Sanctum's Cavalry branch. Other characters include Galenth Dysley, the ruler of the Sanctum and main antagonist;Cid Raines, a Sanctum Brigadier General in the Cavalry who does not trust the governmentand Serah Farron, Lightning's younger sister and Snow's fiancee.

Story

The playable cast of Final Fantasy XIII. From left to right: Sazh Katzroy, Snow Villiers, Hope Estheim, Lightning, Oerba Yun Fang, and Oerba Dia Vanille

Final Fantasy XIII begins in Cocoon as the citizens of the town of Bodhum are being evicted, or Purged, from Cocoon after coming in contact with something from Pulse. Over the course of the game, the player is shown flashbacks of the events of the previous 13 days, which began when a fal'Cie from Pulse was discovered near Bodhum. Lightning's sister Serah had found the fal'Cie from Pulse and been changed into a l'Cie by it. Lightning and Sazh derail a Purge train bound for Pulse in an attempt to save Serah. In the subsequent battle, Snow leads his resistance group, NORA, to rescue the Purge exiles. Several of them, including Hope's mother, are killed. As Snow heads to the fal'Cie Anima to save Serah, he is joined by two of the exiles: Hope and Vanille. The two groups meet at the fal'Cie, and find Serah just as she turns to crystal. Anima then brands them all as l'Cie and they are cast out into a different part of Cocoon. During this transformation, the newly crested l'Cie all have the same vision: a monster called Ragnarok. The group, arguing over the ambiguous nature of the dreamed Focus, find Serah in her crystallized form; Snow remains with her as the others leave.
Snow meets Cid and Fang after being captured and detained aboard the airship Lindblum. Meanwhile, the others escape from PSICOM, but are separated during an air strike; Hope and Lightning travel to Palumpolum, while Sazh and Vanille travel to Nautilus. In Lightning's scenario, she unintentionally supports Hope's goal of killing Snow as revenge for his mother's death. In Vanille's scenario, Sazh discusses how his son Dajh was turned into a l'Cie by a Cocoon fal'Cie and was taken by PSICOM to discover his Focus.At Palumpolum, Lightning tries to persuade Hope not to go through with his revenge and meets Snow and Fang. Fang reveals that she and Vanille were l'Cie from Pulse who were turned into crystals; they were turned back into humans 13 days prior to the start of the game, sparking the Purge Hope attempts to murder Snow, but after Snow saves him from an airstrike, he decides not to go through with it. The party then escapes the city with Cid's aid. At Nautilus, Vanille reveals herself to Sazh as a l'Cie from Pulse, and indirectly the reason that Dajh was turned into a l'Cie. PSICOM then captures Sazh and Vanille and detains them on board the airship Palamecia.
The other members of the party stage a rescue mission and reunite with Vanille and Sazh before they confront Galenth Dysley, the Sanctum's Primarch. Dysley reveals himself as the Cocoon fal'Cie ruler Barthandelus. He tells them that their Focus is to transform into the beast Ragnarok and slay the sleeping fal'Cie Orphan, who keeps Cocoon afloat above Pulse. Slaying the fal'Cie Orphan will result in the destruction of Cocoon. The party escapes and learns from Cid that the fal'Cie believe that Cocoon's destruction will summon the Maker, the creator of the worlds. The fal'Cie cannot harm Orphan themselves.Vanille and Fang reveal to the party that they were involved in the War of Transgression centuries prior, and that their Focus then had been the same: to transform into Ragnarok and attempt to destroy Orphan. The party flies away to Pulse and travels to Oerba, Vanille and Fang's hometown, where they hope to learn how to remove their l'Cie marks. They are unsuccessful, and Dysley confronts the group again. He tells them that he is forcing Cid, now the head of the Sanctum, to create chaos in Cocoon to force the Cavalry to attack Cid and Orphan in a coup d'état.
The party infiltrates Cocoon with the goal of preventing its destruction. They head towards Orphan only to find that the Cavalry have been turned into Cie'th. The party encounters Dysley and overpowers him, but Orphan awakens and merges with Dysley, then compels Fang to finish her Focus as Ragnarok while the others are seemingly transformed into Cie'th. The group reappears in human form, preventing Fang from transforming. The party engage and defeat Orphan and escape Cocoon, which is now falling towards Pulse. As the rest of the party turns to crystal for completing their Focus, Vanille and Fang remain on Cocoon and transform into Ragnarok together. They prevent a collision between Cocoon and Pulse by turning themselves into a crystal pillar between the two worlds. The rest of the party awaken from their crystallization on Pulse and find their l'Cie brands gone. The game ends with Lightning and Snow reuniting with Serah and Sazh reuniting with Dajh.