To get the cheat codes screen, press Left + Y at the Soul Fighter logo. Enter the options menu, then exit.

View credits	AAAAAA
Extra health	ABXXYA
All weapons	XAAYBB
Level 2	AABXYA
Level 3	XAYAAB
Level 4	YYBAXA
Level 5	BABXXY
Level 6	XAXBYY
Level 7	ABXBYB
Level 8	YBBAXY
Level 9	BYAAXB
Level 10	XABBAX
Level 11	YBYXAB
Level 12	XBAXBY