Okay that one I remember. Head to the Outskirts and head to the Beach. From there you just search her Cottage again, and from there the Mansion.
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I'm at the point where I'm locked in the city still though, and one lead is to find Lyx, the other I need her XD
You are trying to track her down in the city. First you find notebooks that lead you to the school, there you find sand, but the areas available don't have sand as far as I can tell.
As far as unlocking the city I have the West side unlocked, and I need Lyx to unlock the East side
Edit: I think I found it! I hadn't unlocked the old club yet. Hopefully that is what I was missing.