Pvt. Sanchez first appears 30 miles west of Billings, Montana with the United States Army as they fight off the Chimera. The battle was turning in the Army's favor when Sanchez was approached by an officer who tells him to report to an installation in Keystone, South Dakota. Sanchez points out that Keystone is in Chimeran Territory to which the officer replies "not all of it".
Sanchez does go to Keystone and finds the base that he meets Cpl. Dashiel, having heard that Sgt. Hale was a Sentinel he starts calling him a "Half blood creep" only for Hale to enter the room. Hale tells them that he specifically requested both Sanchez and Dashiel only for Sanchez to interrupt by saying that the mission was suicide after Hale tried to persuade Sanchez into doing the mission Sanchex then insults him for being half Chimeran to which Hale responds: "I didn't ask to become a Sentinel and you didn't ask to be here but like it or not we're infiltrating the conversion center near Salinas and extrating Dr. Silbermann before the Chimera find the new inhibitor."
Sanchez with no choice of declining joins them but still in the mission call Hale a "Stink" to which Cpl. Dashiel replies "Part Stink". Hale then says that the Chimera were there to which Sanchez seeing the place deserted disagrees only to have to run into cover and the Chimera ambush.
After defeating Hybrids and Grey Jacks the three follow some Grims and Carriers to a conversion center. After entering the conversion center through a sewage pipe they find a room full of spinner pods. Hale opens one to find Dr. Silbermann who tells him to get the inhibitor then kill him. Just as they are about to they are attacked by Spinners who manage to infect Sanchez and mortally wounded Dashiel. Hale who manages to kill all of the spinners turns around to see Sanchez who comments about being able to feel the virus inside of him. Hale gets the inhibitor and plans to inject it into Sanchez to save him but upon realizing this Sanchez tells him not to think of it and then repenting his previous negative views on Hale and the Sentinels, stating that he believes that humanity really needs them before telling Hale to kill him before he turns. Hale agrees by saying:
"There's enough monsters in this world. I won't let you become one of them."
- Sanchez's name was going to originally be Diego as seen here.