Debuting in the story "Kitsune," (Usagi Yojimbo#32, 1992/Usagi Yojimbo vol. 7: Gen's Story), we're first introduced to Kitstune as a street performer who demonstrates tops to enthralled audiences; her real goal is to lower the defences of the crowds so she can pick the pockets of whomever she judges to be carrying a purse. "After all," she says, "a girl has to do what she can to get by."
In this first tale, it's Usagi himself who unwittingly becomes Kitsune's mark; still, despite his initial naivete, he later has enough on the ball to pick Kitsune's pocket and steal his purse back. In her second appearance, Kitsune encounters Usagi while he's travelling with his frequent companion Gen; Gen takes an immediate interest in Kitsune, but Usagi neglects to inform Gen of Kitsune's true occupation, allowing Usagi a rare opportunity to one-up Gen (who typically finds means to trick Usagi into paying all of his bills).
It's quite a bit later when we finally learn Kitstune's background, narrated by her to Gen in "Kitsune's Tale" (Usagi Yojimbo#52, 2001/Usagi Yojimbo vol.16: The Shrouded Moon). There, we learn how her stepmother sold her to work at an inn; when faced with the prospect of being sold from the inn to a brothel, she ran for her life and eventually met Sachiko, a professional thief. We learn "Kitsune" is the name Sachiko gave her (explaining why Usagi's tales could include both the supernatural being Kitsune and a woman simply called "Kitsune").
Under Sachiko's guidance, Kitsune learned how to lift money and create diversions; working as a team, the duo prospered for some time, until they made the mistake of robbing a samurai who had already witnessed one of their earlier robberies. The samurai murdered Sachiko, leaving Kistune by herself. In the wrap-around to this tale, Kitsune meets Kiyoko, an orphan girl who tries to rob Kitsune & Gen. Reminded of her own childhood, Kitsune takes Kiyoko under her wing as an apprentice thief - and thus, the two have appeared together in Usagi Yojimbo since then.
Kitsune is always an interesting character to revisit as she almost always has enemies following in her wake requiring Usagi's intervention. At times she's been considered a romantic interest for Usagi (and Kiyoko for Usagi's son Jotaro), but would the honour-bound Usagi ever seriously romance an unapologetic thief and liar? Kitsune creates fine complications in Usagi's life; she's easily one of the top supporting players of the series.