First, I will show you how the Kart setup works. A character can not go in a kart that is smaller then them. L stands for light character, M stands for medium and H for Heavy. Thus,
L+L=Light Kart L+M=Medium Kart M+M=Medium Kart L+H=Heavy Kart M+H=Heavy Kart H+H=Heavy Kart
Now here are all of the characters, their descriptions, natural partners*, special items and weight classes.
Koopa- Koopa is in many of Mario's games, usually playing some little role or a tiny enemy of some sort. He is a light weight character who is great for begginners.
Natural Partner-Paratroopa
Special Item-Three Green Shells
Paratroopa-Paratroopa is nearly the same thing as Koopa. Except that Paratroopa can fly! He is also light weight and great for begginners. Natural Partner-Koopa Special Item-Three red shells
Diddy Kong-Diddy Kong is Donkey Kongs partner in most games. Diddy Kong is light weight and unlike most characters, has his natural partner in the heavy weight class. Natural Partner-Donkey Kong Special Item-Giant Banana
Baby Bowser-Baby Bowser is not exactly popular because of Bowser's big roles in all of the games. However, Baby Bowser did play the villian in Yoshi's Story and a few other games. He is light weight and like Diddy Kong has a heavy weight natural partner. Natural Partner-Bowser Special Item-Bowser Shell
Toad-Toad makes his appearence in many Mario/Luigi games and in the newer ones(Luigi's Mansion) tends to be a bit of a scaredy cat. He is locked and is light weight and has a good kart for begginners. Natural Partner-Toadette Special Item-Golden Mushroom
Toadette-Not much is none about Toadette. I'm assuming that she is Toad's wife or something like that. Her weight class is light like Toad's and, like all light weights, good for begginners. She is locked like toad. Natural Partner-Toad Special Item-Golden Mushroom
Baby Mario-Baby Mario is a fairly new character, appearing the Mario Golf games. He is light weight and has an odd baby carriage kart. Natural Partner-Baby Luigi Special Item-Chomp on a Leash
Baby Luigi-Baby Luigi is fairly new like Baby Mario. He has another fairly odd kart. Natural Partner-Baby Mario Special Item-Chomp on a Leash
Mario-Mario is the star of this game(Mario Kart), as he is in many others. He is a medium weight character. Natural Partner-Luigi Special Item-Fire Ball
Luigi-Luigi is Mario's younger, taller brother. Normally overlooked because of his brother, Luigi got his star rule in Luigi's Mansion. Natural Partner-Mario Special Item-Fire Ball
Peach-Princess Toadstool AKA Peach, is the subject of some Mario games, being captured in games like Mario 64. Natural Partner-Daisy Special Item-Protection Heart
Daisy-Daisy appeared recently in titles such as Mario Party and Mario Golf. She is a Medium weight character. Natural Partner-Peach Special Item-Protection Heart
Yoshi-Yoshi is a Dinosaur who stars in a handful of Mario Games. He also has his own games such as Yoshi's Island and Yoshi's Story. Natural Partner-Birdo Special Item-Homing Egg
Birdo-I really don't know much about Birdo. I'm assuming that she's Yoshi's wife or something like that. She is alos medium weight. Natural Partner-Yoshi Special Item-Homing Egg
Waluigi-Appearently, Nintendo decided Luigi needed a rival. The product-Waluigi! Waluigi is a fairly new character. Unlike is obese partner, Wario, he is a medium weight character. Natural Partner-Wario Special Item-Bob-Omb
Bowser-Bowser plays the main villian in just about every Mario game imaginable. He is a heavy weight character. Natural Partner-Baby Bowser Special Item-Bowser Shell
Donkey Kong-D.K. also plays the villian in many the mario game. He is a heavy weight. Natural Partner-Diddy Kong Special Item-Giant Banana
Wario-This guy is fat! He plays Mario's rival in many games. Why does it seem all heavy weight characters are evil.... Natural Partner-Waluigi Special Item-Bom-Omb
King Boo- King Boo is one of Mario's many foes. He is King of the Boos(obviously!) and is locked in the start of the game and until near the end. Natural Partner-Petey Piranha Special Item-Random Special
Petey Piranha- I used to think a piranha was a fish, until I unlocked this character! He is some type of cannabalistic flower that is butt-ugly. Natural Partner- King Boo Special Item-Random Special
*Natural Partners do not have to go with eachother. They are just the two that seem to go hand in hand.