Nissa has been transferred to the institution the is run by the lightwood’s, she has known she is a shadow hunter and she is really good at it . Alec, Jace,Isabelle and clary doesn’t know she is coming and they don’t know her ,only maryse knows she is coming because she send for her. Nissa was born with angel blood tasked with protecting humanity from demons . So her and clary are the same.