This unnamed female sniper appeared in Full Metal Jacket as the final and most significant enemy faced by Joker and his allies in the film's climax. She hides in a wrecked building and attacks them from a distance. The sniper manages to kill three members of the squad before Joker reaches her, which is when her identity as a fragile female is revealed. He tries to shoot her, but his rifle jams, but another person from Joker's squad shoots the sniper successfully before her bullets can hit Joker, mortally wounding her. She looks up at Joker from the ground, begging to be put out of her misery, and he does so regretfully while his heartless allies congratulate him for the kill.
It can be assumed that this girl was forced to fight as a sniper or decided to do so because her family had perhaps been killed. Many underage individuals, females, and underage females did fight for the Viet Cong in real life, and having to kill them traumatized many, similar to Joker.