GhostWhen Serah becomes a l'Cie prior to Final Fantasy XIII, it is a pivotal point of the personal stories of Lightning and Snow. It is the worst possible thing, the only certainty that they will lose Serah forever no matter the outcome - it is the end of their insular, personal worlds. Yet, however small, there is a hope, which Snow nurtures until eventually even Lightning concedes and is able to believe. Serah always remains their driving force, their unified reason.
The thing is, that's two people out of a party of six. Serah might be the cataclysm that binds the party together - the person that binds them all together - and their (eventual) figurative inspiration, but they don't know her. Serah's fate matters, because despite being thrown together by circumstance, the party truly bond and care about the others with them on this journey, even after it is over.
But they do not know Serah herself.
Serah is a l'Cie. At their first meeting, Sazh options to kill her, however subtly it's shown - that's what you're supposed to do with l'Cie. You eliminate them. Hope blames Serah for the party being turned into l'Cie - and he's correct, in that if Serah had never disturbed Anima, this whole chain of events would never have happened. Serah, as an individual, means very little to the remaining four party members beyond Vanille's guilt. What do Sazh and Hope care, to begin with? Fang isn't even present. It does not matter that Serah is gone - it might be sad, and they might relate as they have also lost people over the course of the Purge, but Serah is no ghost to them, just a name.
Two years, or five hundred years later, it's another matter.
When Serah dies at the end of Final Fantasy XIII-2, everyone is ruined.
Serah dies and the world loses its Champion. She saves the future, accepts her fate... only the world is not saved, because Caius had a fallback. But it is not those unseen faces of Cocoon and Gran Pulse that Serah haunts. Serah dies, and her companions and friends mourn, and she becomes a figurative ghost. Of course she does. In the time that has passed since Ragnarok, she bonded with the party she never got to function in, becomes the seventh member - as Yeul says, “Etro saved the six, and the one” - and has become more than just a name. She is a person, not an ideal. She has her own ties, and her death irrevocably impacts the cast of Final Fantasy XIII, and not for the better.
As she was the catalyst to bring them together, the loss of Serah drives what remains of the cast further apart.
Five hundred years is a very long time. At first they can pretend, but many ageless years on, her loss shapes the three leaders of this world as hope begins to wither. Snow is never the same. Serah's death completely breaks Snow, little by little. Death is - was - such a final thing and immortality denies him the reunion he wants. He wasn't there, and should have been. He removes himself from the picture and embraces becoming a cie'th. Hope, too, so removed to begin with, is not - Serah's loss moves him as much as Lightning, Vanille, Fang. Sazh hides from the world. Mog blames himself. Noel blames himself further - even as the last of his kind in a dying world, isolated and alone, Noel is never as defeated and broken as he is in the future of Lightning Returns. He withdraws and disappears entirely. He killed the Goddess. He killed Serah. It's his fault.
Where once she was the glimmer of hope, now Serah is the reminder of their pain, the symbol of loss. She is always there, even if they know she is truly gone, her soul cast adrift.
And when Lightning wakes, she is without emotions, because it hurts too much to bear.