She is in the epilogue of City of Glass. “‘Magnus stood in the shadow of a tree, talking to a girl in a white dress with a awan of pale brown hair. She turned as Magnus looked toward them, and Clary locked eyes with her for a moment across the distance that separated them. There was something familar about her, though Clary couldn’t have berkata it what it was.’” Why Tessa is there, whether she is immortal atau a ghost, and why she is talking to Magnus, is a mystery at this time that will be sorted out in later books.
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