Kathryn Janeway with a replicant of Boothby
The Boothby replicant was the leader of a group of Species 8472 that, in 2375, established a training facility somewhere in the Delta Quadrant, recreating Starfleet Command, Starfleet Medical, and Starfleet Academy in perfect detail. He assumed the role of Boothby, the curmudgeonly groundskeeper at Starfleet Academy in San Francisco on Earth.
They intended to instruct members of their species to act as Humans and other Alpha Quadrant species in order to infiltrate the Federation, which they saw as a threat to their existence. He met Commander Chakotay of USS Voyager when he transported aboard the facility to investigate it. Captain Janeway later negotiated a cease-fire with the Boothby replicant, and persuaded him that Starfleet had no intention of invading their realm. Species 8472 received information about the nanoprobe weaponry and Species 8472 canceled their planned infiltration of Earth. The Starfleet crew were allowed to inspect Species 8472 technology and visit their simulation of Earth. (VOY: "In the Flesh")
Apocrypha
In Star Trek Myriad Universe: Infinty's Prism, it is revealed that unlike inhabitants of this dimension, denziens of fluidic space have only one quantum signature, meaning that they do not have alternate dimension counterparts. However, upon being "redesigned" for war in the "normal" universe, they begin to have counterparts from various dimensions returning home. The Boothby double is one such duplicate, attempting to prove that the counterparts can still serve a useful purpose. He equates the experience to wanting to go home and tell war stories in your favorite bar, only to find yourself already there doing so. He once again brokers a peace with an alternate dimension Janeway.