Mario Tennis has a wide selection of characters with the two that need to be unlocked in two categories. Here's a quick run through the preferences and types.
Left handers are mirrors of right handers. For example left topspin can be weak for the right hander compared to a strong left topspin for the left hander. It may be confusing, but can be handy depending on your abilities.
All characters benefit from enhancing their abilities. This can be done by playing through the first three tournaments with any character to receive a star for that character. You can use it to deal with much tougher opponents than before in the last three tournaments.
This is a basic differentiation between Mario and his friends and enemies. With the size, shape and strength of the cast, you can experiment how the playing style of each works.
Mario and Luigi are the only all round characters in the game. They are good choices for beginners getting to grips with Mario Tennis.
Peach, Daisy, Toad and Waluigi can hit the balls to the edge of the court with pinpoint accuracy. Beginners can choose those characters if they like to handle single matches.
Baby Mario, Birdo and Yoshi can move around the court with ease. Learn how to judge the speed of the ball with these characters, and you'll be a match for anyone.
Bowser, Donkey Kong, Donkey Kong Jr and Wario are monsters with their powerful hits! However, with their power comes incredibly slow speed, so you'll need to be a good judge of your opponent's abilities in order to make use of their strength.
Shots fired by Boo, Paratrooper or Shy Guy are wildly unpredictable, as they zig-zag from left to right in the air. Get lots of practice with these characters, and master their wild shots!