Android 16:
In Dragon World, defeat Android 16 as Goku.

Android 17:
In Dragon World, defeat Android 17 as Piccolo.

Android 18:
In Dragon World, defeat Android 18 as Krillen.

Android 20:
In Dragon World, defeat Android 20 as Goku.

Cell:
In Dragon World, defeat Cell as Goku.

Dabura:
In Babidi's Ship, get over 100 Kili.

Frieza:
In Dragon World, defeat Frieza as Goku.

Future Trunks:
In Dragon World, defeat Vegeta on Namek with Kid Trunks.

Ginyu:
In Dragon World, defeat Ginyu on Namek as Vegeta.

Gocule:
Get both Hercule's and Goku's Breakthrough Capsules.

Gotenks:
His capsule is accessed once you enter the Hyperbolic Time Chamber during Dragon mode.

Hercule:
In Dragon World, defeat Fat Buu as Saiyaman.

Kibitoshin:
Gather all seven Dragon Balls. Then summon Shenron and you will have acquired his capsule.

Kid Buu:
In Babidi's Ship, get over 3,600 Kili.

Majin Buu:
In Babidi's Ship, get over 1,200 Kili.

Majin Vegeta:
As Goku, beat Majin Vegeta the first time you fight him.

Nappa:
You must first unlock Vegeta and restart as him. Then in Dragon World, defeat Nappa as Vegeta.

Potara:
To get the Potara Capsule for Vegeta, you must wish for it from the Eternal Dragon Shenron at the end of Dragon World.

Raditz:
In Dragon World, defeat Raditz as Goku.

Recoome:
In Dragon World, defeat Recoome as Goku.

Saiyaman:
In Dragon World, defeat Cell as Adult Gohan the first time you see him.

Super Buu:
In Babidi's Ship, get over 2,400 Kili.

Supreme Kai:
In Dragon World, defeat Supreme Kai as Goku.

Teen Gohan:
In Dragon World, defeat Cell for the last time on Supreme Kai's planet with Adult Gohan.

Tiencha:
Get both Tien and Yamcha's Breakthrough Capsules. A capsule will be given you that explains it.

Vegeta:
In Dragon World, defeat Vegeta as Goku on Namek.

Vegetto:
In the city, when Supreme Kai tells you about the Potara earrings, go to him to get them. When Vegeta comes, give him the other earring.

Videl:
In Dragon World, defeat Super Buu (as Gohan absorbed) as Hercule.

Yamcha:
In Dragon World, defeat Nappa as Tien.

Thanks to Revolution reader Tiny Tim, Clawviper, and Randy!