A Switch Of Twins (Sequel to Shadowed Light)

Everything was going good since Will's shadow coma. Nico and Will's relationship was thriving, until one day Will disappears from his bed. In Will's place is a daughter of Apollo who looks a lot like Will and is his age.
Who is this girl?

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(When it doesn't say a POV it is Nico)
Please read Shadowed Light before this. The story would probably make a lot more sense if you read that first.

I own Cakey and the mystery girl.
@Demigod_HorseLover9 owns Demi.
@BridgetDiAngelo owns Bridget.
@KaitlynMoore573 owns Skylin May
@AmyMayDaughterOfZeus owns Amy May
All other characters belong to Rick "Troll" Riordan.