Play as Bosses
These two codes must be performed at the title screen (the screen with "press start" printed)

Sonork: L, R, Top C, Bottom C, Left C, Right
Demitron and Sonork: A, B, R, L, Bottom C, Top C.

All Character Endings
All codes must be performed at the title screen (the screen with "press start" printed)

Aaron: Up, Left C, R, Right, Down, R, R, Left C
Demonica: Up, Left C, R, Right, Down, R, R, Top C
Demitron: Up, Left C, R, Right, Down, L, L, Bottom C
Eve: Up, Left C, R, Right, Down, R, R, Right C
Gore: Up, Left C, R, Right, Down, R, R, Bottom C
Morphix: Up, Left C, R, Right, Down, R, R, B
Nikki: Up, Left C, R, Right, Down, R, R, A
Scarlet: Up, Left C, R, Right, Down, L, L, Left C
Sonork: Up, Left C, R, Right, Down, L, L, Top C
Zenmuron: Up, Left C, R, Right, Down, L, L, Right C