The former soldier does have a gentle heart, though her harsh mannerisms and speech can leave a cool impression.
Lightning was a former Sergeant of Cocoon’s Guardian Corps. But in order to save her sister Serah from her l’Cie fate, she resigned. From there, she sought to rescue her from the Pulse fal’Cie.
She and her sister are orphans. After the death of her parents, she changed her name to Lightning:
Lightning: I was just like you. My parents died. I have to be strong for Serah, so I thought I needed to forget my past. And I became Lightning. I thought by changing my name, I could change who I was. I was just a kid. Lightning. It flashes bright, then fades away. It can’t protect. It only destroys. Serah tried to tell me, but I wouldn’t listen. I threatened her. The only one who believed her was Snow.
Her hardened demeanor led her to ignore her sister while taking on many burdens. Cosmos calls Lightning to World B during the 12th cycle. It’s unclear when Cosmo called her. From a developmental standpoint, Lightning doesn’t arrive at World B during the events of Lightning Returns: Final Fantasy XIII. The latter released nearly three years after Dissidia 012 Final Fantasy. From a narrative standpoint, Lightning personality isn’t flat in World B. Unlike her appearance in Nova Crystallis 500 years later, Lightning has more of an attitude:
Lightning: “Don’t lump me with Laguna! Like I’ll get lost that easily.”
How about during the events of Final Fantasy XIII-2? While both games released in 2011, Lightning doesn’t show a significant change of character. She also doesn’t wear her Etro’s Knight outfit. We can rule out the possibility that she fell into an abyss by a manifestation of chaos, caused by a paradox
Cosmos most likely called Lightning sometime after branded a l’Cie. We never see Lightning’s l’Cie brand. However, since she can use magic, she still has l’Cie powers.