Gallery:
To unlock the character's galleries, just complete either Tournament modes (Team or Battle Royal), and your winning character's picture gallery will be unlocked.

Unlocking Secret Characters:
This is a list of which characters unlock the others. All of these characters are obtained in the Boss mode by beating the game, and can be unlocked in either the multiplayer or single player modes. Row "a" is the one you must play, while "b" is the character that is unlocked:

a.)           b.)
Brimstone -   Overtkill
Al Simmons -  Skeleton I
Jessica -     Green Beret A
Sam -         The Mafia
Redeemer -    Hell Spawn
Overtkill -   Army A
Tiffany -     Angels E
Grace -       Soleil
Clown -       Vindicator
Tremor -      Cy-Gor
Spawn -       The Curse
Twitch -      Soldier I
Cogliostro -  Vandalizer
Cy-Gor -      Wolf I
Soleil -      Angel Teneres
Vindicator -  Vacillator
Vandalizer -  Vaporizer
Hell Spawn -  Gatekeeper
Gatekeeper -  Zombie I

Boss Characters:
There are four more characters to be unlocked, but they can only be unlocked in the multiplayer mode. In order to unlock these, you must defeat the Boss mode without continuing! The number of times you do this determines which character will be unlocked. Which characters you play to do this doesn't matter. However, I suggest using two of the strongest characters. The Gatekeeper & Gatekeeper team worked great for me!

One time - Spawn I
Five times - Dark Ages Spawn
Ten times - Spawn IV
Fourteen times - Admonisher

Note:
You'll notice that if you get passed the Gatekeeper level without continuing, a new Boss level appears. Instead of fighting Malebolgia at level 12, you'll fight Angel Teneres. Then, level 13 is Dark Ages Spawn. Finally, level 14 has you fighting Malebolgia.

Black Brimstone:
To play as Black Brimstone beat the game with all characters then Highlight Brimstone and hold the two back triggers and press A. Viola! Black Brimstone

Thanks to Revolution reader Darth Paul and Razza!