Very little is known about Luis's early background, other than he was convicted for an unspecified crime. Though he is clearly frightened of prison and proclaims his innocence to Reid, it is currently unclear if he was actually the perpetrator or not. His knowledge about what happens inside a prison, as well as the inmate hierarchy, also implies that he was imprisoned before.
Luis is among the inmates riding a bus heading to Millburn Correctional Facility. Luis starts hitting his head against the window, crying and saying that he doesn't want to go to prison. Reid, who is sitting directly behind Luis, warns that he will give himself a concussion if he keeps doing that. Luis replies that it beats what they will do to him inside. They both introduce themselves, and Luis asks Reid what he was convicted of. Reid replies that he is innocent, and Luis says that he is too. After arriving, Luis and Reid, along with the other inmates, are given prison clothes. Luis is next seen being assigned to a bed in a waiting room by Officer Lionel Wilkins. Later, he approaches Reid in the prison yard, asking him if he has a plan and warning that there are inmates who are coming for him.
Luis is next seen coming out of his cell along with Reid and other inmates. While they work in the laundry room, Reid notices wounds on Luis and asks him what happened, but Luis gives a vague response. Reid asks if there is anything he can do to help, and Luis replies that he does need help. Later, Luis is seen sitting in the cafeteria when Frazier and Duerson approach him and hold him at knifepoint with a shiv as a means of testing him. Later, they beat him to the point where Luis had to be sent to the infirmary.
In the laundry room, Luis thanks to Reid for letting Frazier and Duerson beat him instead. Then, he warns him that the inmates will keep coming at them, but Reid says that they will have to keep fighting. Luis pessimistically replies that they will just end up in the infirmary again, or in the morgue. The next day, he tells Reid that the inmates are expecting him to carry out a task, and that he is scared for his life. At the end of the episode, Frazier and Duerson ambush Reid and Luis in the laundry room. Frazier tells Reid what to do for him in regards to a shipment of drugs coming into the prison. Afterwards, Frazier slashes Luis's throat as a warning and both inmates leave. Reid tries to stop the bleeding and yells for help. Unfortunately, Luis dies off-screen.
- For reasons unknown, the guest cast for Alpha Male credits him as "Randall Harris", even though he was referred to as Luis Delgado throughout the entire episode and in following episodes. The guest cast for Assistance Is Futile fixes this.