Katrine was the identity of Captain Janeway in the French Resistance holodeck simulation created by the Hirogen.
She was the owner of Le Coeur de Lion, a cafe in the town of Sainte Claire which acted as a front for the local French Resistance cell that Katrine led. Katrine, like Janeway herself, was commanding and authoritative. And, just as Janeway and Seven of Nine frequently disagreed, so did their respective characters in the program. Similarly, Katrine and the bartender of her nightclub were old friends and confidants, just as Tuvok and Janeway had been.
Through the use of an implanted neural interface, the crew of the USS Voyager believed that they were the characters.
Janeway's real identity was restored by The Doctor; however, she continued to play along with the simulation, unbeknownst to the Hirogen. After disabling the crew's neural interfaces, Seven of Nine and Captain Janeway were able to stage a counter-strike which allowed Janeway to defeat the leader of the Hirogen and return control of the ship to the Starfleet crew. (VOY: "The Killing Game", "The Killing Game, Part II")
According to the Star Trek Encyclopedia, 4th ed., vol. 1, p. 302, "Genevieve" was a pseudonym used by Katrine.
In the VOY Season 4 DVD extra Braving the Unknown: Season Four, David Livingston likened the character to Humphrey Bogart (possibly in reference to his character in Casablanca, who was also a World War II era nightclub owner).