Lumina speculation

[Goldenboko]

Perhaps she is Serah's "chaotic" side, and the real Serah is somewhere else or something. After chaos flooded the land, maybe Serah's good and bad personalities split or something, and were reincarnated as two separate people. Who knows.
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^ This

And she's working/ helping with the main villain of the game.

They DID drop the bomb near the end that each Yeul reincarnation had her own unique attributes...one liked to sing, another liked flowers, etc...perhaps this is an extension of that.

I'm just getting to the part that Yeul is shown over and over and I can't make heads or tails of it!