Edith Prior, formerly known as Amanda Marie Ritter, is related to the main charcter, Beatrice "Tris" Prior.
Edith was on Tris' dad's side. She was an aunt and her brother carried on the Prior name. Even though "Edith" is an Abnegation name, Edith was actually an Erudite.
Caleb points this out and also notes that many people of the Prior family were also Erudite. There were seven generations from Edith to Tris, according to Caleb in Allegiant.
At the end of Insurgent, Tris and Tobias find a secret file containing a video of Edith. This video is what Tris has been trying to gain control of in Insurgent.
She says that the Divergent are needed to fight a war outside the fence, and that they can help people on the outside. Later on they find out this information was only partially correct.