"Nurse" (Dr.) Cain Diaz first appears in Season four episode 1 "Recall." He is a traveling nurse who first works with Dr. Shannon Rivera. In this episode, he often encounters trouble with Kenny, who reminds him he is just a nurse.
When a patient is admitted to the to the ER with an apparent PCP overdose, who says he didn't take any drugs. The patient is an avid cellist and says that he would never risk his gift for drugs. Shannon is skeptical, Cain believes him and argues in his favor.