Cline's father and grandfather had fought the Hath.
Cline was one of the first people to greet the Doctor and his companions upon their arrival on the planet Messaline. The Doctor showed Cline his empty hands to prove that he was unarmed. However, when Cline saw the Doctor's hands were clean, i.e. didn't have a skin sample taken from them, he forced the Doctor to process his DNA. He made the Doctor put his hand into a machine which took a skin sample, analysed the DNA of the skin cells and manipulated them to create a new DNA pattern which was then used to create a new soldier, Jenny, who was technically the Doctor's daughter as she was made from the Doctor's DNA.
Cobb put the Doctor, Donna and Jenny in a cell because he didn't want the Doctor, Donna or Jenny spreading treason. Cline was given the job of guarding them. Jenny tricked him into opening the cell by kissing him through the bars and stealing his gun. Cobb later found Cline tied up, gagged and embarrassed. Cline joined the search for the Source. After Jenny's death, Cline asked the Doctor to let them give her a proper ceremony. After the Doctor, Donna and Martha left, Cline, alongside Hath Gable, witnessed the mysterious resurrection of Jenny. (TV: The Doctor's Daughter)