Although Remy is the only character normally playable throughout the main campaign (side missions not withstanding), Ratatouille technically has 26 characters in total, all stored internally within the game's files waiting to be called for their purposes within the various modes. By going into Menu.tsc, editing the assigned character listing for each stage, and activating the debug tools, it's easily possible to play as any of these 26 characters in any level- all with varying degrees of functionality.
P_REMY_R: Seems to play identically to normal Remy. The only noticeable difference is that his tail swipe is bugged, as his tail curls inwards when he attacks rather than swinging outwards.P_REMY_S: Semaphore Remy seen at the end of The City of Lights and Little Chef - Big Kitchen. He can't do much besides walk, slide and jump on slippery spills, and take water damage.P_REMY_T: Contrary to the character name, this isn't actually Remy, but Serge as seen in the heist in Little Chef - Big Kitchen. He is also limited to walking, slipping, and falling into water.P_LINGUI: This may look like the Linguini you play as during the cooking minigames in the kitchen, but it turns out that this is actually an unused, older version from earlier in Asobo's part of development, and can be found in use in their prototype. The main difference between both Linguinis is their walk animations and the fact that Remy is not visible through his toque.P_CELI_M: Surprisingly, a fully playable Celine with a full set of animations and Remy's abilities. She appears like this for player two in most multiplayer minigames, but the mere fact that the entire single player campaign works as her is incredible.P_Emile: Remy's brother Emile as seen in the Rat Race multiplayer minigame. He's about as playable as Serge, but due to being needed for Rat Race, Emile fully functions in any raft found in the game.P_AICELI: The computer AI controlled version of Celine for Rat Race. Due to being built for the CPUs, the player can do nothing with her.P_AIEMIL: The computer AI controlled version of Emile. Doesn't function either.P_AITWIT: The computer AI controlled version of Twitch. Guess how playable he is.P_R_SWMK: Celine as seen in the conveyer belt switches minigame in The City Market. Has as much autonomy as Remy's switches version.P_R_WCCT: Twitch, from the two wire connecting minigames in the courtyard and the market. Can't move, unfortunately.P_FISH: Funnily enough, this isn't even actually a living character- it's the fish hook controlled by the Wii remote in the Wii exclusive fishing minigames. It can't budge an inch, and in order to even see it, a coordinates modifier is required.P_L_WASH2: Linguini's arms, once again holding a hose, as seen in the Lean, Mean, Dishwashing Machine Wii exclusive minigame. It appears to be identical to the first hose version at face value, so the necessity for two versions is currently unknown.