From Nookipedia, the Animal Crossing wiki
Tom Nook (たぬきち, Tanukichi) is a tanuki, or Japanese raccoon dog, that owns and operates the town shop (except in New Leaf, where he runs Nook's Homes). As soon as the player moves in to town, Tom Nook will offer part-time employment at his shop, in order to pay off their newly acquired house mortgage.
After spending a certain amount of money in the shop, he will remodel to include a wider range of items. In all versions of Animal Crossing series, the final shop expansion, Nookington's department store, will include a second floor with Tom's nephews, Timmy and Tommy (except New Leaf). In Wild World, Harriet's salon, Shampoodle, will also be included.
At the beginning of the game, Tom Nook owns a small wooden shack. As the player buys items from him, the store will expand, eventually into a two-story supermarket. There are a total of four expansions, beginning with Nook's Cranny, which upgrades to Nook 'n' Go, then to Nookway, and lastly Nookington's.
In Animal Crossing, Tom Nook runs a lottery on the last day of every month where Spotlight Items and NES games can be won if the player has raffle tickets. There are also random sales on occasion, as well as a selection of grab bags on Sale Day.
This is the only game where the player can give passwords to Tom Nook. This allows for the transfer of items between towns.
His store is now always adjacent to the Able Sisters in this game, and the lottery, password and Sale Day features are removed. He also no longer sells clothes or umbrellas. However, the Tom Nook Point System is introduced to allow for eventual discounts on all purchases, with occasional Point Special days awarding five times the points, and Spotlight Items will infrequently appear in store. Nookington's contains Shampoodle on the lower floor, and upgrading to this store triggers Tom Nook's episodes.
Each player now shares Tom Nook's mortgage, and Tom will still force new players to work for him even when it is fully paid off. He states that he will 'not charge' the player for the lessons they are learning in Animal Crossing life.
His picture can be obtained by giving him a Safe, and the quote is "I'll be there with bells on! Ho ho!"
Tom Nook acts similarly in Animal Crossing: City Folk to Animal Crossing: Wild World. His store is never directly to the west of the Able Sisters, however, and his picture cannot be obtained in this game. But, players each have their own mortgage once again. Tom Nook will erect a flag outside a house with its mortgage fully paid off.
After a period of time with Nookington's in City Folk, Tom Nook will ask the player a question to "better serve his customers". Based on the answer given, Nookington's can close and reopen as a previous store to better suit the player's needs. Periodically, after the first questioning, Tom Nook will ask again, and the player can once more change the store.
The suggestions and answers are:
In Animal Crossing: New Leaf, Nook runs a real estate store instead, and players no longer must work for him. The player, the new mayor, moves in without a home, but Tom Nook assists the player by letting the player determine the location of their new home. Construction of the home, however, takes a day and a 10,000 Bells down payment from the player, so they must sleep in a tent. Once the home is built, players must pay off the debt before they can make any expansions. After this, Tom Nook offers exterior home decorations and encourages the player to sign up for the Happy Home Academy, an association that scores players' homes.
In Animal Crossing: Happy Home Designer, he is usually referred to as King Nook. He helps run the monthly Happy Home Network Design Contest. During the contest, the player will be asked to design a home for King Nook. His appearance is notably different, as he wears royal clothing including a crown.
Tom Nook makes an appearance in the Villager's Final Smash, Dream Home, and makes an appearance as a trophy. The trophies have a different design and description from one another.
Wii U version
Tom Nook appeared in Doubutsu no Mori (film) as the town shop owner, just as he does in the games.
Sometimes, upon entering his store, he will appear in thought, and may tell the player a story when approached:
Tom is very money-driven, and many years ago left behind his friends, most importantly Sable, to travel to the big city. While there, he has dreams of success which are quickly broken by the harsh life there. After making nothing of himself, he returns to the player's town to set up his store. Nook no longer appears to be friends with Sable, though the two often mention each other to the player. In New Leaf, he even appears to have a slight crush on her, as shown when he blushes when Lyle mentions her and her sisters.
Nook seems to have a varied reputation amongst the villagers, with some speculating that he is, in fact, a man in a suit. His large, bushy tail seems popular amongst lazy people. Many will say that he has a checkered past, hinting at illicit activities. He is openly derided by Redd as a "raccoon in an apron."
Nook has been thought to have worked with Redd in the past. This is further evidenced in Animal Crossing: Happy Home Designer, where, if you have built his house, he may sometimes say, "Have you ever worked with a fox? Yes, foxes are never to be trusted, hm? I worked with a fox once in my life. Never again. Not after...the incident. Stop! I cannot say a word more! Let us assume it was tragic and not speak of it again."
Tom Nook's description in the City Folk's game guide:
In Happy Home Designer
Names in other languages