Android #16:
Have Goku in the first position of your team when you fight Android #16 and Android #18 and defeat them.

Android #17:
Play as Android #20 and defeat Android #17.

Android #18:
Have Krillin in the first position of your team when you fight Android #16 and Android #18 and defeat them.

Android #19:
Play as SSJ Vegeta and defeat Android #19.

Android #20:
Play as Android #19 and defeat Android #20.

Burter and Jeice:
Play as Goku and defeat them.

Captain Ginyu:
Play as Vegeta and defeat defeat Ginyu-Goku and Jeice.

Cell 2:
Play as Cell's first form and defeat Android #17.

Cell Jr.:
Play as Perfect Cell and defeat Cell Jr.

Cell:
Play as Piccolo and defeat Cell's first form.

Chibi (Little) Trunks:
Unlock Majin Vegeta.

Evil Buu (Gohan absorbed):
Unlock Evil Buu (Gotenks absorbed), then play as SSJ Gohan and defeat Evil Buu (Gohan absorbed).

Evil Buu (Gotenks absorbed):
Unlock Evil Buu, then play as Gotenks and defeat Evil Buu (Gotenks absorbed).

Evil Buu:
Play as Fat Buu and defeat Evil Buu.

Evil King Piccolo:
Successfully complete the Future stages as SSJ Future Trunks.

Fat Buu:
Play as Young Trunks and defeat Fat Buu.

Frieza:
Play as Super Saiyan Future Trunks and defeat Frieza.

Future Android 17 And 18:
Win the fight in EX Story1 as Android 19 And 20.

Ginyu-Goku:
Play as Captain Ginyu and defeat Goku.

Goten:
Win the Goten vs. Trunks battle.

Guldo:
Play as Krillin and defeat Guldo.

Hercule:
Unlock all characters in the game, then when you are in Story 22, win the fight versus Trunks with the Ginyu Force (once with Guldo, once with Recoome, once with Burter, once with Jeice and once with both Ginyu forms). Hercule will now be unlocked.

Nappa:
Play as Piccolo and defeat Nappa.

Perfect Cell:
Have Cell 2 in the first position of your team when you fight Android #16 and Android #18 and defeat them.

Pure Evil Buu:
Unlock Evil Buu (Gohan absorbed), then play as SSJ Vegeta or Fat Buu and defeat Pure Evil Buu.

Recoome:
Play as Goku and defeat Recoome.

Trunks:
Lose the Goten vs. Trunks battle.

SSJ4 forms:
Although SSJ4 was not featured into Dragon Ball Z, it is possible to get this form of Vegeta, Goku and Gogeta (Goku and Vegeta fused by Fusion Dance). Unlock Goku, Vegeta and Veggetto at their strongest forms (Goku SSJ3, Vegeta Magin Vegeta and Veggetto SSJ). Then, complete the entire game including Trunks' Extra Story with only those three characters at their strongest form in any order.