There's an unused, colorless model of a Cactuar within the game's data. A similar looking metallic Cactuar made it into Final Fantasy XIII-2.
Contributed by ProtoSnake
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Within the game's data is a naked model of Vanille. While not uncommon to have naked models of characters to use as bases, what makes this model obscure is that the developers gave the model nipples, which are normally not included. The model is likely used during the scene in which Spoiler:Vanille transforms from a crystal back into a human. However, during this scene, she is covered.
Contributed by Dazz
Early in development, Lightning was meant to be a flirtatious character with more sex appeal. To make Lightning a more serious heroine in the vein of previous heroes, Cloud Strife and Squall Leonhart, Fang's character - which was originally a man - was rewritten as a woman, and some of Lightning's traits were transferred to her. Not only this, but her original silver hair color was given to Hope Estheim in favor of pink.
Contributed by Ricardoo_Suarez
Serah was planned to be the 7th member of the team, but due to issues during development, she was turned into an NPC. The idea of making her playable turned into a reality with Final Fantasy XIII-2.
Contributed by gamemaster1991
According to the Final Fantasy XIII Ultimania Omega, at one point in time, Vanille was considered to be the official main character. The idea was scrapped as the development team had already released a trailer and art featuring Lightning taking the role.
Contributed by gamemaster1991
Lightning's hairstyle resembles the shape of the crystallized Cocoon. This can also be seen in the game's logo.
Contributed by MegamanOmar
The number 13 occurs in many ways when playing the game. These are a few examples:
• The game consists of 13 chapters.
• The flashbacks take place 13 days before the events of Chapter 1.
• There are 13 shops to buy items from.
• When clearing certain missions, you can get an Analog, which there are 13 of.
• The Datalog has 13 submenus.
• To get a 5 star rating you need to get 13,000 battlepoints.
Contributed by aapjes