From Nookipedia, the Animal Crossing wiki
Totakeke "K.K." Slider, (とたけけ, Totakeke), or K.K., is a dog who has appeared in all the Animal Crossing series games to date. His main role is to play and sing songs for the player. He made a cameo appearance in Super Smash Bros. Brawl and in Doubutsu no Mori (film). He was also one of the three Animal Crossing characters to be a trophy in Super Smash Bros. Melee, the others being Mr. Resetti and Tom Nook. He has recorded a performance called K.K. Slider: Live at the Roost. There are a total of 55 songs in the original Animal Crossing, 70 songs in Wild World, 75 in City Folk, and 91 in New Leaf. See the full article here.
K.K. Slider has a stereotypical laid-back musician's nature, which is reflected in his mellow speech towards the player. He comes across as someone who wants to be liberated, as shown by his rebellion against putting a price on his songs. This adds to the musician stereotype of going against a particular norm. K.K. seems to have an obvious passion for performing for others, as he offers to play for people for free every Saturday.
In Dōbutsu no Mori and Animal Crossing, K.K. Slider starts the game up for the first player. Every Saturday night from 8 P.M. to 12 A.M., he can be found sitting on a wooden stool outside the Train Station. If spoken to, he will perform a song for the player and, after finishing, give them a copy to take home.
K.K. Slider still has the same role as in the previous installments, though he has added several new songs to his repertoire.
In Wild World, K.K. Slider still sings and plays for listeners, performing at the same time (every Saturday from 8 P.M. to 12 A.M.) as he did in the previous games, but now he plays them at The Roost, on top of a stage located in the cafe's upper left corner. The first time K.K. Slider is encountered, he will play a random song for the player; however, on all subsequent visits, the player can request a song or refuse to be played to. After the performance, whether the song was random or requested, he slips a free bootleg of the song into the player's pockets, informing the player that he is opposed to the industry practice of putting a price on music. Compared to previous entries in the series, several new songs have been added. In addition, giving him a country guitar will allow the player to obtain his portrait. If the gates are open, K.K. will not appear until they are closed, as he cannot perform in the multiplayer mode.
K.K. Slider has the same role in Wild World, but now he gives encores. The player can tell him exactly what song you want to hear, or let him pick. Another way to choose a song is to tell him a mood, and he will pick something out based on it. Cranky villagers will sometime mention that they think that Phineas is K.K.'s day job. As with Wild World, he will be in The Roost starting at 8pm to Midnight and some new songs have been added. However, the player will not receive a bootleg copy of the encore. The multiplayer issue in Animal Crossing: Wild World is fixed and he now appears in in Animal Crossing: City Folk and onward during multiplayer
K.K. Slider once again appears but this time in a separate building called Club LOL. Now he will perform remixed NES tunes and some of his tracks for the player to listen to including several new songs. His appearance has changed, he now wears glasses, a nice cap, and has headphones. Every night at 8 p.m., K.K Slider will play disco music at Club LOL, complete with Villagers dancing and a large LCD screen in the back displaying pictures. On Saturday nights at 8 p.m., as in previous games, he will play songs by requests, mood, or randomly, depending on what the player wants. Attending 20 of his acoustic performances in New Leaf will have K.K. Slider give the player his picture. K.K. Slider's songs can be bought from T.I.Y., T&T Emporium, or the catalog for 3,200 Bells each.
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