Here is a list of unlockable characters
Adult Gohan ----- Fight Evil Buu
Android 16 ----- Put Goku in your team when fighting Androids 16 and 18
Android 17 ----- Defeat him as Android 20
Android 18 ----- Put Krillin in the first position when you fight Androids 16 and 18
Android 19 ----- Defeat him as SSJ Vegeta
Android 20 ----- Defeat him as Android 19
Cell ----- Beat the first form of cell with Piccolo
Cell 2 ----- Defeat Android 17 with the first form of Cell
Future Android 17 and 18 ----- Win fight in ex. story 1 with Android 19 and 20
Majin Vegeta As Super Saiyan2 ----- Goku, lose to Majin Vegeta once, fight him again and he'll be unlocked under Vegeta.
Perfect Cell ----- Put Cell 2 in the first slot for the fight against androids 16 and 18
Captain Ginyu ----- Defeat Ginyu Goku and Jeice as Vegeta - NOTE: You can have Anyone help you in this fight
Ginyu Goku ----- Defeat Goku as Captain Ginyu
Guldo ----- Defeat Guldo as Krillin
Recoome ----- Defeat Recoome as Goku
Burter and Jeice ----- Defeat them as Goku - you can have Y.Gohan help you in this fight
Cell Jr. ---- Defeat him as Perfect Cell
Evil King Piccolo ----- Beat the Future Stages with only SSJ Future Trunks and beat the game, he will be availeble in the next storymode.
Fat Buu ----- Use Young Trunks(normal)to beat fat buu. A blue screen comes up saying you unlocked him. NOTE: If you unlock Young Trunks the same time through you have to beat the game again.
Future Trunks ----- When Training with Goku in the Hyperbolic Time Chamber, go to the jar in the bottom left corner, and grab whats in there. Then, even though it says its empty, just keep looking, and you'll get a Escape Trunks card. Then just get to the Cell Games.
Goku Super Saiyan 3 ----- To unlock him in story 30 on the first time through the game, unlock ALL of the Z Fighters up to story 30 and play Future Trunks' story.
Goten ----- Win Goten vs. Trunks
Gotenks ----- Have SSJ Goten and SSJ Young Trunks. Play Trunks' Ex Stories and read all the fliers on the wall. At the last screen after beating the game, press start rapidly.
Nappa ----- Defeat Nappa as Piccolo
Piccolo-Kami ----- Play through the game as normal until you get to Cell's first form.Use Piccolo and lose once then continue and Piccolo-Kami will be on your character select screen.
Super Saiyan 2 ----- Gohan For Adult Gohan SSJ2 you will have to win the fight in story27 with Adult Gohan SSJ.
Trunks ----- Lose Goten vs. Trunks
Vegeto ----- Have SSJ Goku and SSJ Vegeta. Play Trunks' Ex Stories and read all the fliers on the wall. At the last screen after beating the game, press start rapidly
Vegeta Normal On Namek ----- when the Namekian attacks you, pick left. He'll be unlocked right before you face Ginyu and Jeice. Or...talk to Vegeta twice looking for Android 20, then speak to Tien.
Vegeta Super Saiyan ----- After you unlock Vegeta Normal, he'll be selectable at the Android Saga.
To play the Future Trunks Story you must have Future Trunks unlocked, but when Cell comes back after self destructing, use SSJ Vegeta and fight him, if it's right, a little dialogue will appear after Cell and Vegeta fight. Then after, you'll enter the +1 and +2 stories.
Here is a list of all the unlockable Evill Buu forms (Note: You must unlock these in order):
Evil Buu ----- Defeat as Fat Buu
Evil Buu - Absorbed Gohan ----- Defeat as SSJ Gohan
Evil Buu - Absorbed Gotenks ----- Defeat as Regular Gotenks
