"Sage" is the nickname of a Ghost Lead. He preferred the nickname to the regular Ghost Lead callsign.
In 2019 he and his team of Nomad, Weaver, and Joker were sent to Ukraine to extract a deserter. However, it was a trap set by Vympel and the mission went catastrophically wrong. Sage and Joker were wounded during the extraction, with Sage critically so.
Nomad was then assigned to Venezuela with his own team.