Atticus Liddell was a doctor who Jack Bristow put under Witness Protection 25 years before Season 4. He later returned to save Jack's life after he suffered from radiation poisoning.
Before Season 4
25 years before Season 4 Liddell wrote a thesis regarding the treatment of genetic mutations. In 1981 he left the United States to pursue his theories through dealing with a nuclear accident in the Soviet Union, specifically Minsk. However, Soviets became convinced he was a spy for the U.S. government, which he was. One of Jack Bristow's handlers informed the government of the suspicion and Jack was able to extract Liddell before they could act.
Jack put Liddell in Witness Protection and gave him a new identity, moving him to Finland. Back then Jack had a strict policy not to keep any records of anything as sensitive as Liddell's location. The handler - the only other person besides Jack Bristow to know where Liddell was - died eight years before Season 4, leaving Jack as the only person knowing Liddell's true location (Mirage).
Season 4
Jack had hallucinations of Liddell during the later stages of his radiation poisoning. In his delusion, Jack imagined Dr. Liddell knowing Sydney since she was a child, and inserting things under Jack's skin to test the progression of the poisoning (A Clean Conscience). In actuality, Jack was in a building on Pierpont where he was injecting himself with a deadly poison (Mirage).
APO managed to track down Dr. Liddell by convincing Jack that it was 1981, the day before Liddell was hidden. They found him in Helsinki, where Sydney approached him and told him that her father, Jack Bristow, was in trouble. He was brought to APO, where he was greeted by Arvin Sloane in a manner that suggested they were familiar before Liddell's move. Liddell was taken to see Jack in the APO infirmary, and told him that he would be able to make him better (Mirage).
Relationships
Jack Bristow
Liddell clearly felt strongly for Jack, who saved his life in the Soviet Union. When Sydney told him that Jack needed his help, Liddell came instantly, saying "After what Jack did for me..." (Mirage).