The codename of the first female G.I. Joe introduced in 1982 is Scarlett.
Scarlett, whose real name is Shana M. O'Hara, is one of the original members of the G.I. Joe Team. She debuted in the Marvel comic G.I. Joe: A Real American Hero #1 in June 1982 and was also released as an action figure that year.
She was the first female action figure in Hasbro's A Real American Hero toyline. Her specialties include counterintelligence, martial arts, acrobatics, and weapons training, and she is especially associated with a compact crossbow. In the original continuity, she also develops a close relationship with Snake Eyes.
Jinx, Cover Girl, and Lady Jaye are other female G.I. Joe characters introduced later. Scarlett's name is occasionally presented as “Agent Scarlett” in later toy releases, but Scarlett remains the canonical codename asked for here.