What is the alias of the undercover agent?
Arthur "Cody" Jarrett is a ruthless, psychotic criminal and leader of the Jarrett gang. Although married to Verna, he is overly attached to his equally crooked and determined mother, "Ma" Jarrett, his only true confidant.
Cody and his gang rob a mail train in the Sierra Nevada mountains (referred to as "the tunnel job"), killing four members of the train's crew. While on the lam, Cody has a severe, instant migraine, which Ma nurses him through. Afterward, Ma and Cody have a quick drink and toast, "Top of the world!" before rejoining the others. The gang uses the cover of a storm to change hideouts and split up. 
Informants enable the authorities close in on a motor court in Los Angeles where Cody, Verna, and Ma are holed up. Cody shoots and wounds US Treasury investigator Philip Evans and makes his escape. He then puts his emergency scheme in motion: confess to a lesser crime (the "hotel job") committed by an associate in Springfield, Illinois at the same time as the tunnel job - which was federal crime - thus providing him with a false alibi and assuring him a lesser sentence. He flies to Illinois and turns himself in, where he receives one- to three-years in state prison. This ruse does not fool Evans, however, who plants undercover agent Hank Fallon (aka prisoner Vic Pardo) in Cody's cell in the Illinois State Penitentiary. His task is to find the "Trader", a fence who launders stolen money for Cody. Hank's angle is to become a surrogate "ma" to Cody and get him to talk.
Vic Pardo