Commander Valerie Archer was the identity assumed by a member of Species 8472 in the mid-2370s in a training simulation designed to accurately simulate Earth and Starfleet Headquarters. (VOY: "In the Flesh")
Following their encounter with the USS Voyager in 2374, a group of Species 8472, on space platforms called terraspheres, created a simulation of Starfleet Headquarters on Earth. The polymorphed members of Species 8472 were conducting a role-playing mission to better understand their enemy, the Federation and Starfleet. Valerie Archer was one of the officers impersonated by Species 8472.
The impersonated Archer took up many interests in humanoid culture, including such reading material as Beyond the Galactic Edge, Humanity's Quest for Infinity and A Cave Beyond Logic: Vulcan Perspectives on Platonic Thought. She was also partial to Klingon martinis.
In 2375, Chakotay, posing as Commander Jason Hayek, encountered Archer on Terrasphere 8. Chakotay attempted to discover what Species 8472 was planning by developing a rapport with Archer. During their first meeting, an officer began to morph back to his original form. Archer asked Chakotay whether he had ever reverted. She also told him that she disliked breathing oxygen and sleeping as if she were Human. Later, on a date with Chakotay, she gave herself an isomorphic injection in the thigh in order to maintain her form. Chakotay discussed with her whether it was necessary to invade Earth, and Archer stated that Humans had allied themselves with the Borg and must be destroyed. Archer wanted to give Chakotay a goodnight kiss, wondering what it would be like. When she kissed him, she rubbed her hands on his face. A scan of her hand revealed that Chakotay was Human.
After Captain Kathryn Janeway and Species 8472 reached an agreement and settled their differences, Chakotay and Archer kissed once again. He was impressed with her mastery of the art of kissing. (VOY: "In the Flesh")
Valerie Archer was portrayed by actress Kate Vernon.
According to Nick Sagan, "I really wanted to name the character Archer as a homage to Dave Bowman of 2001 and to my father's character Ellie Arroway [from Contact]. You put bow and arrow together and you get Archer."
It is unclear exactly if there was an actual Valerie Archer in Starfleet, or if the identity was created by Species 8472. Realistically, if Species 8472 wished to infiltrate Starfleet it would be easiest to swap out existing officers with their own replicas. According to the Star Trek Encyclopedia (4th ed., vol. 1, p. 41) states that there was an actual Archer, and that Species 8472 re-created a replica of her for the simulation.