Big heads:
Hold L + R at the title screen, and choose a character with those buttons held.

Fight as Bosses:
Press Up, Down, X, B, A, Y, C, Z, Start at the title screen. The Boss characters will now be accessible from the character selection screen. To access the Bosses normally:

Gaia
Finish the game normally

Sho
Finish the game in story mode.

Cupido
Enable Sho. Highlight Sho on the character selection screen and press Up + A.

Alternate costume colors:
Hold Down and press A, B, or C when selecting a character.

Adjust camera:
Pause the game. Highlight any selection except "Exit" selection on the options menu. Hold L, highlight the "Exit" selection, and press R. Control the camera with the following actions, then press Start to return to the menu.

D-pad: Moves camera
L: Rotate left
R: Rotate right
Y: Zoom in
Z: Zoom out

Quick special moves:
Rapidly tap C or Z during game play.