Big Boss, formerly known as Naked Snake, was the founder of the elite black ops unit called FOXHOUND. He would learn a majority of his techniques and training from his legendary mentor The Boss, as well as develop CQC with her in the 1950s. Disgusted with his government after the events of Operation Snake Eater, Big Boss would retire from war.
After the San Hieronymo Incident, Big Boss became one of the founding members of The Patriots, but would leave due to Major Zero's actions, becoming a mercenary. It was at this time he met with Kazuhira Miller in Colombia while on a mission, eventually forming the Militaires Sans Frontieres (MSF), with Miller becoming the second in command.
This would lead to the events that happened in Ground Zeroes, with their Mother Base being attacked by XOF forces. In their escape, Paz would jump out of their chopper, revealing that there were two bombs inside her. The resulting explosion would send their chopper on a crash course with the XOF one. This crash would lead to Big Boss ending up in a coma for nine years, beginning the events of The Phantom Pain.