From Nookipedia, the Animal Crossing wiki
Isabelle (しずえ, Shizue) is a dog that appears in Animal Crossing: New Leaf to help the player with their duties as mayor. Her name was first revealed as Hisho during the New Leaf developer commentary, then changed in the June 2012 Nintendo Direct to Shizue. When localized to English, her name became Isabelle. It may be based on the fact that the top of her head resembles a Bell sack.
Because she was the first new character to be revealed to players, many people speculated that she is the 'eager secretary' that will assist the player in his/her new role as the town mayor. This was later confirmed when she was shown talking to the player, with the name Hisho (ひしょ), which is Japanese for secretary, during Tokyo Game Show 2011. She is said to be very clumsy and very absent-minded. She has a twin brother called Digby who runs the Happy Home Showcase.
Isabelle makes a cameo appearance in the first E3 trailer of Super Smash Bros. for Nintendo 3DS and Wii U. Isabelle appears in the game as an Assist Trophy.
Isabelle, along with villager, appears as a playable racer in the second Mario Kart 8 DLC Pack, which will be released in May 2015.
Isabelle's main role in Animal Crossing: New Leaf is to act as the player's secretary and to help them with their mayoral duties. At the start of the game, she helps the player settle in by offering them advice, as well as providing them with optional tasks akin to those given by Tom Nook in previous games. In addition, she also helps the player earn their Development Permit, which allows for the enactment of ordinances and the construction of public works projects, by helping them attain a 100% approval rating and, when the player's house is built, making them a resident of the town. When the player gains the abilities to build public works projects and enact ordinances, she assists the human character with those as well, including the ceremonies when projects are complete.
She spends most of the time in the town hall, but is also the main character to stand in the plaza during events. During such events, the town hall is closed, preventing the player from performing their mayoral duties.
Isabelle takes over Mr. Resetti's role of reminding the player about saving until the Reset Center is built. She may be caught sleeping in the town hall a week after the first public works project is built, and this will allow the Dream Suite to be built on Main Street.
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