In Animal Crossing: New Leaf, a character has been hanging out in the shopping district for a few days. She has never lived here, there is no campsite available and she is not a specialty character. Will she ever leave?
In Animal Crossing: New Leaf, the game stores 16 different non-residing villager characters. These stored characters will show up in the shopping district area, stores, etc. Usually, these stored characters are all past residents of your own town, but occasionally you can pick up the character information from another town you've connected to. Rarely, they can be characters randomly chosen to wander your game.
Will these wandering animals ever leave? So like I said, the game stores 16 characters. 16 unique characters. This means you can rotate her out of the stored characters by allowing current villagers to move, eventually bumping out the unwanted character.
I've used this technique to get back one of my favorite villagers, who accidently moved out. Villagers that are on this 16 character list will not move back into your town. I bumped him out of the 16 (basically out of the game's memory of the character) so I could adopt him from a friend's town when he was in boxes.
The game has this weird system to make the in-game relationships more realistic. How weird would it be if a villager moved out your best friend and moved back in 3 days later, introducing themselves like they've never met you?