So I was looking over things on twitter and City of Glass and came across this passage,

Alec glanced toward the mini-forest, the colored lights reflecting in the blue irises of his eyes. Magnus stood in the shadow of a tree, talking to a girl in a white dress with a মেঘ 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 familiar about her, though Clary couldn't have ব্যক্ত what it was. Magnus broke away and came toward them, and the girl he'd been talking to slipped into the shadows of the trees and was gone. (pg. 525 in CoG)

So I have been doing a lot of research on who this mystery girl is and I came across Cassie's সাধারণ প্রশ্নাবলী where she confirms this to be in fact Tessa Gray from Infernal Devices

Why Tessa is there, Cassie says whether she is immortal অথবা a ghost, and why she is talking to Magnus, is a mystery at this time that will be sorted out in later books.

My guess is that she is immortal though I do not think that she is in fact a warlock mainly because I think she is the relative of someone really important. My theory would be either Jace, Clary, অথবা one of the Lightwoods. I think this because Clary say that Tessa looks familiar, familiar as in see has seen her somewhere. Like in her peers. But what is bugging me is that out of all of them, Tessa looked at Clary. I have two theories on this,

1.Clary is holding Jace's hand who is a relative of Will
2.Clary is a decedent of Jem

I really like the #2 theory because it is rather ironic the Clary and Jace would be in প্রণয় with each if their who-knows-how-many-greats grandparents were best friends. I think that would be very interesting. But this will be clear in the future because I have a feeling Miss Gray will appear in the series once again. What do আপনি guys think? Do আপনি think I am off base অথবা are আপনি as excited as I am with this new discovery?