"Beta" was the callsign of an unnamed, highly-trained operative stationed at the top-secret installation known as the Guest House. It was located in an abandoned World War II bunker.
After Skye was shot by Ian Quinn in Italy, and was operated on at the Trauma Zentrum, Agent Coulson and his team were told that she would not survive long, due to the extent and severity of her injuries. Coulson went on a search to find the location where he was treated after being killed by Loki. Fitz and Simmons discovered the location of the Guest House. Beta and his partner, Alpha were stationed to guard the bunker from any unauthorized entry. When Coulson's team arrived at the Guest House, and infiltrated it after but being unable to give the proper countersign, Beta and Alpha went on the offensive and attacked the intruders with lethal intent. Alpha was shot and mortally wounded by Agent Ward while Beta was shot and killed by Agent Garrett.