To unlock the following characters you should complete the corresponding goal:
Android 16: Defeat Android 16 in Dragon World with Goku.
Android 17: Defeat Android 17 in Dragon World with Piccolo.
Android 18: Defeat Android 18 in Dragon World with Krillen.
Android 20: Defeat Android 20 in Dragon World with Goku.
Cell: Defeat Cell in Dragon World with Goku.
Dabura: Get over 100 Kili in Babidi's Ship.
Frieza: Defeat Frieza in Dragon World with Goku.
Future Trunks: Defeat Vegeta on Namek in Dragon World with Kid Trunks.
Ginyu: Defeat Ginyu on Namek in Dragon World with Vegeta.
Gocule: Get both Hercule's and Goku's Breakthrough Capsules.
Hercule: Defeat Fat Buu in Dragon World with Saiyaman.
Kid Buu: Get over 3,600 Kili in Babidi's Ship.
Majin Buu: Get over 1,200 Kili in Babidi's Ship.
Majin Vegeta: Defeat Majin Vegeta the first time you fight him with Goku.
Raditz: Defeat Raditz in Dragon World with Goku.
Recoome: Defeat Recoome in Dragon World with Goku.
Saiyaman: Defeat Cell in Dragon World with Adult Gohan.
Super Buu: Get over 2,400 Kili in Babidi's Ship.
Supreme Kai: Defeat Supreme Kai in Dragon World with Goku.
Vegeta: Defeat Vegeta on Namek in Dragon World with Goku.
Yamcha: Defeat Nappa in Dragon World with Tien.