Nothing is known about Castro's life prior to or as the outbreak began, except that he resided in San Diego with his wife and daughter and served as a member of the California Army National Guard.
Sergeant Castro is one of the Guardsmen stationed inside the safe zone where Madison's neighborhood is located. He is one of the guardsmen to help detain Griselda Salazar and Nick Clark and bring them to the medical facility for observation.
When Ofelia Salazar confronted the guardsmen regarding her mother, Sergeant Castro moves in to restrain her. Andrew Adams intervenes and comforts Ofelia, insisting on taking her home. Castro is later seen at the gates with his men. Moyers orders Castro to prepare his men to take Travis to the command center. Castro complains, commenting on the loss of Nevins and lack of rest of his men, but Moyers ignores and mocks him.
After receiving a distress call, Moyers and his squad drive to a library overrun with infected where another squad of their fellow Guardsmen was trapped. Castro orders Travis to stay in the Humvee, and proceeds with his fellow Guardsmen into the zombie-infested building. As Travis listens to the radio, it can be heard that Moyers' squad eventually run into some trouble, probably getting outnumbered, surrounded then overwhelmed by zombies.
A few minutes later, the squad came out without Moyers, also with zombies pouring out from the library door. A Guardsman tosses a grenade, while the squad rushed back to the truck frantically with Richards replying Travis that Moyers "got held up (by zombies) and is not coming back". When Travis reminds him he was supposed to be taken to the command center, Castro ignores and claims he's fleeing to San Diego to find his wife and daughter.
He, along with Richards and Cole, dropped Travis a few blocks from the Safe Zone, and leave to San Diego.
This list shows the victims Castro has killed:
- Numerous counts of zombies and possibly unnamed people
- Castro is credited as Sgt. Castro in all of his appearances.