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 nube 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 dicho it what it was.’” Why Tessa is there, whether she is immortal o 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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