For the majority of All the King's Men, Jack thinks that Ellis, the Scholarly Attorney, is his father. Jack can't get over the fact that Ellis walked on him and his mom when he was a young child, leaving behind the Burden house and fortune to pursue a life of religion. Jack believes that any man who leaves his mother, Mrs. Murrell, is a weakling and a fool. Jack's mother told him that Ellis left "because he didn't love Mother" (3.39). Jack finds this utterly incomprehensible; he can't imagine any man not loving his mother.
Ellis, who creates and distributes Christian tracts, lives simply and helps care for the needy. The people who run the Mexican restaurant below his apartment think he's a saint.
For Jack, Ellis Burden is a continual source of shame and anguish. This shame continues until Jack learns that 1) Ellis is not his father; and 2) that Ellis left because he found out that his best pal, Judge Irwin, was sleeping with his wife. We are 99.9 percent sure Ellis knows that Jack is not his son.
Once we learn about the affair and the betrayal, we realize that Ellis is involved in a pitched battle between the present and the past, between remembering and forgetting, and between shame and truth. Jack's feelings toward Ellis are completely transformed when he makes this realization. The greatest testament to this transformation is that Ellis is living with Jack and Anne. Such a situation strikes a poignant and painful harmony at the novel's end.
Another important thing about Ellis is that he is nephew of Cass and Gilbert Mastern. His mother was their sister, Lavinia. It is through this connection that the Mastern papers come into Jack's hands in the first place. For most of the story, Jack thinks that Gilbert and Cass are his relatives, and that the story of Cass Mastern is his personal family history. This adds to the shame he already feels concerning connection with Ellis.
It is only when Jack is freed from the story as family history, that he is able to begin to understand his own history.