Creation and development
The overarching theme of the games was the effects the deities of the core mythos on the human population, especially the fate that was forced upon the main characters. Yuji Abe, a designer on Lightning Returns, defined it as "A battle with destiny", with the burden of destiny growing progressively heavier for the main characters over the course of the games. The setting and story was written around and drawn from the official mythology for the Fabula Nova Crystallis series, which also includes Final Fantasy XV and Final Fantasy Type-0. For Final Fantasy XIII and its sequels, Toriyama created a story focusing on the existing deities within the mythos and their influence on the world. Different deities from the mythos were focused on in each installment, such as the goddess Etro in XIII-2. One of the defining features of the cast is the game's central protagonist, Lightning. Toriyama wanted Lightning to be an exceptional female protagonist for the Final Fantasy series, with her having great strength and combat ability, as opposed to the gentler figures of Aerith from Final Fantasy VII or Yuna from Final Fantasy X. Unlike those characters, Lightning's personality was conceived well before her outfit was designed or her voice actresses were cast. How the character was portrayed and how her story and personality was evolved became one of the driving forces of the series.
Most of Final Fantasy XIII's characters were designed by Tetsuya Nomura, who also served as the character designer for Final Fantasy VII, VIII and X. The stories of the characters were created by Motomu Toriyama and Daisuke Watanabe. The main story concept was "characters at the mercy of a predetermined, unjust fate". Nao Ikeda designed Snow, Jihl Nabaat and Gadot, Lebreau, and Maqui. The cast, along with the world's characters, were intended to mimic the multi-ethnic feel of the United States rather than Asia or Europe. During production of the first game, Toriyama wanted the cast to be a group that was originally combative with one-another, and designed the game's narrative with several key points which would bring them together: these included the scene between Sazh and Vanille in the city of Nautilus, and the reconciliation between Snow and Hope in the town of Palumpolum. The characters went through several changes in the early stages of development, the two noted examples being Sazh's ethnic origin and Fang's gender.
For XIII-2, Toriyama wanted a dark and serious tone to the world and story, in contrast to the jovial atmosphere of Final Fantasy X-2, and the story was scripted to play out as "pieces of drama" like a television series. Its story and characters were focused around the concepts of mortality and, in Toriyama's words, the "wish for rebirth": the latter theme was directly inspired by the Great East Japan earthquake. Watanabe stayed as one of the game's writers, with writer and novelist Emi Nagashima, better known under her pen name of Jun Eishima, coming on as a story consultant. Lightning's outfit for the game was designed by Isamu Kamikokuryo, who worked from a rough outline done by Nomura, who also designed the characters' facial features. Two new character designers were brought in: Hideo Minaba, who contributed to new character outfits such as Hope and Alyssa's, and Yusuke Naora, who designed outfits for Serah, Noel and Caius. Lightning and Serah's designs were created to directly reflect the environments they start out from. The character of Noel was added at a later stage in the original planning, since the original plan for Serah to travel alone with the moogle Mog seemed to clash too much with the game's serious nature. For Lightning Returns, the developers decided to have Lightning as the sole protagonist so that players could get to know her better. The story was created to bring closure for the characters of the series. The core themes of the game were the "salvation of souls", and the "rebirth of Lightning", the latter being the main reason the game was not called XIII-3. Norura returned to design Lightning and Snow's new looks and Kamikokuryo returned to design new outfits for Lightning and help with the world design. Two new designers brought in were Toshitaka Matsuda, an artist for Final Fantasy XII: Revenant Wings, and Toshiyuki Itahana, who had worked on Final Fantasy IX and the Crystal Chronicles series. Both designed multiple battle costumes for Lightning, many of them inspired by the artwork of Yoshitaka Amano. Itahana and Matsuda also respectively designed Lumina and Bhunivelze. Watanabe returned as the main scenario writer, while also receiving ideas for scenes from Toriyama and other members of staff. His work on the script was slow, causing problems from the rest of the development team: in response to this, he worked hard to create an appropriate finale. During his work, he had a strong nostalgic feeling while writing the characters' lines.
In order to reduce the delay between the local and international releases, the English voice acting for XIII was done while the game was in development. Unfortunately, it lacked the infrastructure needed to support simultaneous development and localization. The lack of deadlines, poor communication and synchronization between the various departments, and continuing changes to the script and to cutscenes led to a turbulent development. One of the most notable knock-on effects was that the game's script needed to be re-translated as the various cutscenes changed, and large parts of the dialogue had to be rerecorded, party because they lacked proper emotional drive at the right moments. For XIII-2, a tool called Moomle, developed by English translator Tom Slattery and his Japanese counterpart Teruaki Sugawara from their experience with the first game, was used to make sure all parts of the localization process were synchronized. For Lightning Returns the Japanese voice actors recorded their dialogue well after their characters' scenes had been created, as opposed to the normal procedure of recording lines before cutscene creation. For the English version, the amount of dialogue translation and recording was so large that there was over two months' delay between the game's local and international releases.
Eidolons (召喚獣 Shōkanjū, Summon Beasts), beings who aid the protagonists after being tamed in battle, are the game's versions of summons. The ones featured in the games are series staples Odin, Shiva, Alexander, and Bahamut, and newcomers Hecatoncheir and Brynhildr. Their summon sequences were designed to be flashy, but also mixed with gameplay: this approach was inspired by comments from players of previous titles who had not wanted to wait as long for the summons to take effect as in previous titles. The Eidolons, which could transform into vehicle-like forms for their masters to ride, were built around the game's theme of "transformation". They were to have been featured in XIII-2 as part of a DLC episode, but the idea was scrapped and the gameplay was folded into one of the title's minigames. For Lightning Returns, although the Eidolons still exist, they were not available in gameplay and merely served as an element of the story. Carried over from the Fabula Nova Crystallis mythos are the fal'Cie, demigods who work to find a way of bringing their respective deities back into the world. The fal'Cie can choose people to become l'Cie, servants given magical powers and a task to complete within a given time, called a Focus. Those who succeed go into crystal stasis, while those that fail become monsters called Cie'th.