Sergio Hernandez was the brother of Jose Hernandez, and was a buyer of the Cordilla Virus on Day 3.
Sergio was broken out of prison by Rick Jackson on Day 3 because Rick was trying to gain Sergio's trust. Sergio was then reunited with his brother, and the three of them went to go buy the cordilla virus from Mitchell Stewart. When Rick and the Hernandez's lost the virus to Abdul Amad, Jose wanted to back out, but Sergio didn't, so Sergio shot Jose and stole the virus from Abdul. But, the virus turned out to be a bomb, that later killed Sergio, because Mitchell Stewart ripped off whoever bought the virus and left with the real virus.