There are five characters which you will get on your team throughout the course of the game. Remember that only three can be on your team at once. Each charcter hs their stats rated on a scale of 1 to 10 [based on them using the Level Up bonuses I reccomend in Basic Hints & Strategy].
Detailed Character Descriptions
How I Came Up With The Stat Ratings

Mario    Mallow    Geno    Bowser    Toadstool


  Mario
Weapon:Punch / Hammer / Shell
Armor: Shirt
Element: Jump/ Fire
Mario doesn't need to much of an introduction, Nintendo's main man is finally starring in his own RPG, complete with all of his Shell kickin', Flame throwin' and Super Jumpin' abilities. He's the most balanced of the characters, having about average stats except for his exceptionally high Hp, which is in fact the highest Hp out of the five [the only character who can get a full 255 Hp]. His attack power though, isn't as high as most people would think.
Overall [without equipment]: 7.3    Overall [with best equipment]: 8.0

  Mallow
Weapon: Glove / Stick / Cymbals
Armor: Pants
Element: Thunder / Ice
Mallow is a young frog [?] who Mario will meet early on in his adventures as Mario helps him recover his Grandfather's lost Frog Coin. Later when you reach Nimbus Land you'll learn more of Mallow's background [and why he doesn't resemble a frog]. He actually has the best Magical abilities in the game [peach has the better healing magic though]. He lacks Hp and Speed, however.
Overall [without equipment]:7.5    Overall [with best equipment]: 8.0

  Geno
Weapon: Arm shot / Shot / Star shot
Armor: Cape
Element: None
Geno is a warrior from afar sent to retreive the fallen pieces of the Star road. He takes the form of a wooden puppet for his stay on earth [being that is normal form is just a little glow of light which it shows at the end of the game]. As surprising as it may be, Geno is all around the strongest character [not counting stat changes from equippment]. He has higher Attack than even Bowser, reaching into the 300's if he has his best weapon and armor. He also has high speed and good magic his only downfall is his naturally low Hp, but his good defense will make up for that.
Overall [without equipment]: 8.8    Overall [with best equipment]: 8.9

  Bowser
Weapon: Claw / Chain hammer / Mario toss
Armor: Shell
Element: Jump [Eaeth] / Poison
Bowser is of course, Mario's sworn nemesis. However this time they're actually working together. Bowser needs his castle back, and it happens that the people who are now controlling his castle are the ones who are after the stars, giving Mario and Bowser somewhat of a common goal. Bowser is the sloest of the characters and has low magical powers, but has very strong Hp, Attack and Defenseive qualities. Oddly enough, despite his sturdy built Bowser has relatively weak weapons and armor.
Overall [without equipment]: 7.9    Overall [with best equipment]: 7.6

  Toadstool
Weapon: Slap glove / Parasol / Fan
Armor: Dress
Element: None [Healing]
Toadstool, the famed princess of the Mushroom Kingdom is once again kidnapped by Bowser. And Mario's quest to rescue her eventually develops into the quest to find the stars. She is the last teamamte to join your party.
Toadstool has the weakest regular attacks [unless you count her strongest weapon] and has very good magical abilities. Her only offensive spell is the Spritz Bomb, which is her final ability and compared to the other character's final attacks is very weak. Her healing spells are extremely strong however.
Overall [without equipment]: 8.1    Overall [with best equipment]: 8.7