From Nookipedia, the Animal Crossing wiki.
Mr. Sonny Resetti (リセットさん, Risetto-san, Mr. Reset), also known as just Resetti, is a mole that appears in all the Animal Crossing series games. His name is a pun on the word "Reset," which is what causes him to appear. His job is to make sure the player saves his/her game. The more the player resets the game without saving, the angrier he gets, getting to the point where he forces the player to recite specific lines word by word. Resetting the game too many times will lead to a "fake" reset. After resetting many times in Animal Crossing: Wild World he will talk about his cousin Vicious Vole Vinnie. His first name is revealed to be "Sonny" in Animal Crossing and Animal Crossing: City Folk by his calmer older brother, Don.
Resetti's role in Animal Crossing stays the same throughout the whole series, but one of the main differences in this game is that he tells the player "Thanks for buying Animal Crossing!" and breaks the fourth wall. As aforementioned, he appears when the GameCube resets or is turned off without saving. After resetting, he appears outside the player's house, and gives a lecture on why saving is important. Resetting constantly will lead to him forcing the player to repeat lines of text to remind them about the importance of saving.
In Wild World, he plays the same role as in the GameCube game. Resetti can also be seen in The Roost of the Museum on Sundays between 2:30 P.M. and 4:00 P.M., drinking coffee with a relaxed, calm attitude. If the player strikes up a conversation with him he will talk calmly about his work, apparently friendly while not on the job. In this game, he never mentions the "resetting", as none of the DS units has a Reset button; instead he says "save the game".
His role in City Folk stays the same as in previous games, but he can now be encountered in a new area in the City, called the Resetti Surveillance Centre, with his older brother occasionally making an appearance at the venue. This appears when, on a random day, going to the city after 8:00 P.M., and going to the right of the City near the tunnel where he may give the player a Silver Shovel to leave the area. Afterwards, he appears the second time the game is accessed by the same player, to talk about the importance of saving, regardless of whether or not the game has been turned off with or without saving. This is the only time he will appear by the player's house for something positive.
 Other appearances
Mr. Resetti appears as an Assist Trophy in Super Smash Bros. Brawl, where he will give the players a long lecture. His dialogue box obstructs either the top or bottom half of the screen. Mr. Resetti will also respond to stage events, such as characters falling asleep or getting knocked out. He doesn't actually help you win fights, he is there as a random annoyance, although attacking him for a while makes him angry and causes an explosion.
 Trophy information
 Parental Guidance in European Manuals
In the European Animal Crossing: Let's Go To The City instruction manual, it states that Mr. Resetti may be frightening and/or disturbing to young children.
"Information for parents - Mr Resetti
It is important that players save before switching off the Wii console in order to retain data after each play session (see Saving Data, above). If players switch off without saving, the character Mr. Resetti may appear upon restarting.
Mr. Resetti's purpose is to teach players the importance of saving. However, parents should be aware that his personality and tone of voice, while intentionally humourous, are authoritative and may be disturbing to young children."
 Iwata Asks: Animal Crossing Q+A
In the Iwata Asks: Animal Crossing question and answer session, the directors of the game admitted that Mr Resetti. was disturbing to children and sometimes made young girls who were playing cry. Thus, they made the decision to have the option to include Mr. Resetti.
 See also