Tobias Eaton is one of the main characters in the Divergent series. From the first book in the trilogy, we know Tobias as Four, Tris Prior's initiation instructor. Originally from Abnegation, he had a terrible childhood. His father is Marcus Eaton, one of the council members of Abnegation, was abusive towards Tobias and his mother Evelyn, who left him when he was a a child. Tobias, was then left with Marcus.
As the series develops, you learn more about his character, especially in the last book, Allegiant, which is partially told in Tobias' perspective.