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 Dōbutsu 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.
When K.K. Slider plays a song in Animal Crossing, sometimes Mr. Resetti will appear in the background. In Dōbutsu no Mori and Animal Crossing he starts the game up for the first player. He also sits on the stool by the Train Station. He will sing songs for the player on Saturday nights and give them a copy to take home. Tom Nook will pay 5 bells for His songs, though they cannot be bought back.
K.K. Slider still has the same role as in the previous installments, though he has added several new songs to his repertoire.
K.K. Slider still sings songs, but now he plays them at The Roost, on Saturday nights from 8pm-Midnight. The first time K.K. Slider is encountered, he will play a random song, however, on all subsequent visits, the player can request a song, or by saying 'Not this time,' K.K. will sing a random song. After the performance, whether it was random or requested, he slips a free bootleg of the song into your pockets, saying "Hey, I gave you a boot. You know, a bootleg. Those industry fat cats try to put a price on my music, but it wants to be free." Some new songs have been added. The player can obtain his picture by giving him a country guitar. or attending 20 of his acoustic performances in New Leaf. If the gates are open, K.K. will not appear until they are closed. This because K.K. 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. There are several new songs. His appearance has changed, he now wears glasses, a nice cap, and has headphones. On Saturday nights at 8pm, as in previous games, he will perform with his guitar and play original songs and requests. 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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