Lumina is a character in Lightning Returns: Final Fantasy XIII. She mysteriously resembles Lightning's deceased sister Serah, but her personality is the polar opposite. She is a mischievous, capricious girl who at times appears to be helping Lightning while at other points she hinders her. She has lived the past five hundred years while crossing paths with old friends of Lightning, including Snow, Sazh, and Vanille, and is at home in the Chaos, able to summon forth beasts from its mist.
Lumina frequently crosses paths with Lightning, even appearing within Lightning's mind as she travels to and from the Ark, where their conversations can't be overheard by Hope and by extension Bhunivelze. She seems to know a great deal about Lightning, using this to her advantage as she alternately taunts and goads her.
It is revealed near the end of the game that Lumina is in fact a piece of Lightning. Specifically, the piece of Lightning's heart in which she wished to keep Serah's memory safe, and that also served as Lightning's emotional core and her softer side, Claire Farron. But because Lightning had worked so hard to close off her heart, both to herself and to others, when she entered crystal stasis, this piece fragmented away, becoming Lumina.
In the end, as Lightning overcomes her internal struggle and accepts the fullness of her heart, she and Lumina become one once more.