|First game||Animal Crossing: New Leaf|
Isabelle was first announced in an Animal Forest: Jump Out (Japanese version of Animal Crossing: New Leaf) promo video, known simply as "secretary". While only shown for a couple seconds, it showed some rough sketches and her in-game model following Villager, presumably for setting up a Public Works Project.
Animal Crossing: New Leaf
In Animal Crossing: New Leaf, Isabelle is among the first people to greet you as you enter town, mistaking you for the mayor. She acts as the tutorial getting you set up as your position as mayor of this town. Afterwards when you become mayor, you talk to her to decide on public works project, change the town ordinance, change the town flag, change the town song and reset villagers' clothes and catchphrase. On special events like, she stands in the town plaza usually in a costume for the holiday and gives you a present for the holiday. She has a twin brother named Digby.
Marketing & Promotion
It didn't take long before Isabelle became one of the most well-known and popular characters in the entire franchise. By pre-ordering Animal Crossing: New Leaf at certain retailers, you could recieve an Isabelle figure along with a Town Hall. Other purchasable toys soon became available in Japan, with around 1/2 of them scheduled to come to (or already in) America. Isabelle was even given a Twitter account, where it is portrayed as if it is really Isabelle, and she makes notes of things like scheduled Plaza Events to this day.
Super Smash Bros.
Isabelle is an Assist Trophy in the game. She was revealed on April 4th, 2014. She heals the one who summoned her by throwing fruit. She was also in the first trailer to Super Smash Bros. for Nintendo 3DS and Wii U as a character in the Villager's town in the trailer's opening cutscene.
"Isabelle is the mayor's highly capable secretary. She can be forgetful sometimes, but you can always count on her for information about the town. She wears her hair up in a bun that makes her look like a shih tzu. Mostly because she is one! She also has a twin brother named Digby."
Mario Kart 8
Along with Dry Bowser and Villager, Isabelle appears as a racer in Mario Kart 8's second DLC pack, Animal Crossing x Mario Kart 8. This marks the first (and currently only) time an Animal Crossing character other than Villager has been playable in any game.