Claire Farron is the younger sister of Sarah Farron- their parents died when they were both young and so the responsibility fell on young Claire to raise her sister. She had to grow up faster and therefore she was forced to let go of her childish leisurely behavior.
"There is no record of when or why she decided to take on the name Lightning as far as I know, I’m guessing it was when she became a soldier. And then that whole fiasco started with her sister being marked by the fal’cie, getting engaged, and completing her focus..and, after that, turning to Crystal.."
Lightning also became marked- after she found her sister, alongside 4 soon-to-be comrades; Upon their attempt to defeat the Fal'cie responsible - they were all marked and cast into a new Focus. After the events that followed, when all was restored, Lightning was wisked away again, she was not rewarded but isolated from the only person she cared about.
She was trapped in Valhalla, with Caius, it wasn't a coincidence Serah fought to correct the timeline with Noel, or that Noel killed Caius..unleashing the Chaos
Lightning became Crystal, preserving the memories of her sister, until the God of light woke her, and made her an "angel of death" to do what needed to be done, "Etro's will" to save souls- the goddess of death was dead but the medium she left unguarded, the Chaos left unchecked...
||I believe that Lumina was born from the Chaos because of Lightning's prayer to preserve Serah's memory, the Goddess Etro aknowledged that||