Edith Prior, formerly known as Amanda Marie Ritter, is the ancestor of Beatrice "Tris" Prior's father, Andrew Prior.
Edith was on Tris' dad's side. She was an ancestor who carried on the Prior name. Even though "Edith" is an Abnegation name, she was actually an Erudite.
Caleb Prior 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 Eaton 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.
Movie Portrayal Edit
Edith Prior is protrayed by Janet McTeer in The Divergent Series: Insurgent.