Instructions
Select the "Star Battle Arena" game mode from the main selection screen. This is a storyline mode where single players battle through all of the opponents to win the "Party." This game mode is the only way to unlock the two bonus characters.
Choose the character with which you want to play. Select from 14 different characters. You have to stick with the same one to go throughout the whole "Star Battle" so choose wisely.
Play through each of the six game boards. These boards include "DK's Treetop Temple," "Goomba's Booty Boardwalk," "King Boo's Haunted Hideaway," "Shy Guy's Perplex Express," "Koopa's Tychoon Town" and "Bowser's Warped Orbit."
Reach the final battle after "Bowser's Warped Orbit" game board. Choose from battling either "Hammer Bro" or "Blooper." This choice is important because the character you battle is the one that opens up after successfully winning.
Defeat the character in the final selection of mini-games. Collect as many coins as possible and battle him until you win. Once you win, the character will be available from the list at the start of the game. Along with him, you will have "Bowser's Warped Orbit" opened for use in all other game modes.
Repeat this process, but once you reach the final battle, select the opponent that you have not opened yet. Upon beating him, he will be available for use throughout the game.