Ginyu: Defeat Ginyu in Goku's DU.
Recoome: Defeat Recoome in Goku's DU.
Frieza: Defeat Frieza in Goku's DU.
Saibamen: Unlocked in the Dragon Arena, Nappa must break in. You will need to fight a character level 31-35 for this random event to happen, and your fighter must be at a lower level than your competitor. You will see an extra scene before the loading scene if he has broken in. If you don't get this, you can simply quit and return to the character select scene and try again until it happens. Beat Nappa to unlock Saibaman. As this is random, it could take any number of attempts to happen, but will eventually.