During the countdown prior to a race, watch for when the third light starts to turn off and the fourth light just starts to glow. Accelerate right at that point and you will get a significant speed boost.
Race the Cyber Field 2 track and collect the blue diamond between the two holes near the finish line. When you finish the race, save the game and Bear will be available in the Character Selection Screen.
First, unlock Bear as above. Using Bear, beat the record time in Ganbare Dochu 1 and 2.
On the Sky Bridge 2 track, collect the Blue Bell powerup. Now you have to get the blue diamond located on a platform to the right of the huge gap jump. To get to the platform, use the Blue Bell powerup as an extra booster when you jump. When you get the blue diamond, and finish the race, save your game. King will now be available in the Character Selection Screen.
On the Moon Road track, collect the Blue Bell power-up. Now look for the very first long jump - you will need to jump this gap using the powerup to land on the platform with teh blue diamond. One you have the blue diamond and finish the race, save the game and Vic Viper will be unlocked in the Character Selection Screen.