Who is Mr. Fitzgerald "Fitz"?

Mr. Fitzgerald or "Fitz" as he is called by Briony was her old tutor, a man that she respected for everything he did except for his strange friendship with Cecil, a known buffoon and overly masculine individual. He has been sent away at the start of the novel by Father, for reasons unknown. Briony later discovers that he was an arsenic user along with Cecil explaining both their friendship and the reason for which he was sent away.