Honestly, I only played FFXIII-2 and the demo of LR, and know Lightning is Serah's older sister. Yet, I don't want to miss this challenge.
Inspiration/Explanation: At first glance of Lightning, I immediately saw her as a strong minded type of woman. So, I thought maybe she could wear something chic. Sketches ranged from a party dress to casual wear. In the end, I had an article that was like a dress hemmed into a tuxedo. I looked into the other garbs she acquires in LR, once I eyed the Duelist garb and it was time to finalize an idea.
The concept of Vidofnir came about as I searched for what color the outfit should be. Hope's weapon in FFXIII has this name, but I've read that Vidofnir is a golden rooster whose loud crow will wake those upon the coming of Ragnorok. Regarding some myths roosters are sacred, messengers, symbols of light and dawn. I chose Vidofnir as it closely relates to FFXIII's inspiration of Norse Mythology.
Weapon: I wanted to work with the idea of speed and precision, so I choose the rapier as her weapon. Like the gold feathers of Vidofnir, it is a rapier made with the same precious metal. Quick with precision in heavy lunges, it is effective close range, and can deal multiple hits at meager damage. Along with the lithe weapon are two throwing blades, mounted above or below the quillon. They'll come back... eventually.
Accessory: beaded pince-nez; red diamond below her yellow bow-tie.