Edith Prior, formerly known as Amanda Marie Ritter, is the ancestor of Tris Prior's father, Andrew Prior.
History
Edith was on Tris' dad's side. She was an ancestor of Andrew Prior and the first on the Prior name.
Prior Family
Caleb Prior points this out and also notes that many people of the Prior family were also Erudite and even a few Candor. There were seven generations from Edith to Tris, according to Caleb in Allegiant.
Insurgent
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.
Divergence
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
Edith Prior is portrayed by Janet McTeer in The Divergent Series: Insurgent.