Mario
Mario was just an ordinary plumber in Brooklyn until he fell down a massive pipe that transported him to the Mushroom Kingdom. It was there that he discovered that a princess had been captured by a monstrous beast, and he set out to save her. Because he saved her, Peach granted Mario access to live in the Mushroom Kingdom, so Mario brought Luigi out of Brooklyn, and they've been living in the Mushroom Kingdom ever since. Anytime Bowser messes with Peach, Mario is always there to go save her. He also loves to do his job of plumbing. Mario has experienced more than you can shake an Italian noodle at; he's saved Peach too many times to count, gone on safaris, ran from ghosts, he even went to space! He's done it all, and Luigi has accompanied him many times. Oh, and he can fly with a raccoon tail (don't ask).
Luigi
Luigi is the reason it's "The Mario Bros." and not just "Mario." Luigi has been on many adventures as well. But, the truth is, Luigi is not as brave as Mario; he's not really the hero type. He easily gets distracted by food and is quite lazy. Once he inherited a mansion, and it was full of ghosts! However, Luigi loves to have a good time; he's always there when it's time for some golf or tennis or karting or even some fighting (Smash Bros). While Mario is the delicious bread of the Italian panini, Luigi is the meat and cheese; he's got a lot of personality. Many Mario fans love Luigi because they can relate to him; he's the underdog, but he's proved to be the unlikely hero more than once.
Princess Peach
Princess Peach Toadstool is the princess of the Mushroom Kingdom. We have no clue who her father is, so don't even ask. She is the ultimate "damsel in distress," holding the record (probably) for "the princess who's been captured by the villain the most" award. And I doubt she will ever be beaten (that Bowser is persistent!). She is very feminine, enjoying the color pink, having tea, floating down gently with her dress (or a parasol), and crying out "Mario!" when she's captured. What does she do most the time? We just told you, weren't you paying attention? Peach is also madly in love with Mario (I think; she kissed him on the cheek once). How could she not love him? He's rescued her so many times! Peach is kind and loving, but she is also blond. Sorry, blond people.
Bowser
Bowser is the King of the koopas. He's a giant turtle with an attitude. He's captured Princess Peach so many times and had his butt stomped by Mario so many times, you wonder why he even bothers (perhaps he does it just to annoy Mario?). Bowser is kind of a clumsy, bumbling villain. Sometimes he can't make up his mind about Mario, even helping him sometimes! Bowser always has evil intentions, but they usually don't work out very well. He has many kids who have done his bidding for him, but they have also failed in defeating Mario (they're a family of losers). I think Bowser has learned to be content with never winning; after all, if you can't beat 'em, join 'em.
Donkey Kong
Donkey Kong is a famous movie star from DK Island. His first feature was called "Donkey Kong" and actually co-starred Mario, known as Jumpman in the film. He's worked with only the greatest movie stars around. Against common belief, Mario and DK are actually very good friends; Mario even invites DK to all of the sport outings (karting, golf, tennis, soccer, even the Olympics). DK bought the island that bears his name after doing several movies; shortly after residing there for awhile, it was overrun with kremlins. So DK grabbed his pal Diddy Kong and they kicked some kremlin butt, even kicking King K. Rool out of the place. This caused DK to gain even more fame. He is big and strong, but has the heart of a gentle baby baboon. What's with the look?
Yoshi
Yoshi goes way back with Mario, even helping Mario when he was just a baby. That's right, Mario was abandoned as a baby and taken in by a dinosaur! How on earth is Mario so civilized? No one knows. Yoshi is a species of dinosaur that is very kind to humans. They fight only to protect their young (Baby Mario), and when it comes to that, they are forced to use their very own eggs as weapons! Yoshi not only protected Baby Mario, but also defeated Baby Bowser! That means that Mario and Bowser go way back too! But how. . . just forget the whole thing. Yoshi comes from Yoshi Island, which is just south of DK Island and Bowser's Domain. The Yoshis have also established a small town in the Mushroom Kingdom known as Yoshi Village (I guess some of them wanted to live in the Mushroom Kingdom instead of Yoshi Island). Yoshi is Mario's faithful companion, always by his side when he is needed. What? The Princess was captured again? Yoshi! Let's go!
Toad
Toads are the little citizens of the Mushroom Kingdom (hence they have mushrooms on their heads). They are all over the place. Sometimes you bend down to pick up a nice looking mushroom when you realize it's just a Toad that's laying in a bush. Oops! Toads are actually very spunky little guys. Toad accompanied Mario, Luigi, and the Prnicess on an adventure once (except it was just in Mario's dream). Then Toad was invited to be in the Mushroom Grand Prix (which turned into the annual Mario Kart Tournament). He was overjoyed: "I'm the best!" Okay, so he can be arrogant sometimes, but then you just step on his head to put him in his place. Toad used to have a nice clear voice, but then he started smoking a pack a day, and now his voice is all raspy. What a shame, Toad, what a shame.