All of these codes will let you be any of the souls in every battle. The Master Code is included and the only thing that will happen is that the mood bar will be messed up if you use the last five and you'll always be evil with the Dark Soul
Master Code 9F6637CD47C3 8BE829F64B66 E8EAE8B3494F Always Roll Soul FB3268957095 Always Guts Soul F0606C973205 Always Wind Soul F36268977005 Always Search Soul FC35EC952A95 Always Fire Soul FF37E8956895 Always Thunder Soul F465EC972A05 Always Dark Soul (rocks!!!) 77E7E8DF4801 Always Proto Soul F767E8976805 Always Metal Soul 7BB268DD5091 Always Number Soul 78B06CDD1291 Always Junk Soul 70E06CDF1201 Always Aqua Soul 73E268DF5001 Always Wood Soul 7CB5ECDD0A91
And don't worry about the Dark Soul, you won't lose HP in any way, however, you can't control MegaMan when he has Dark Soul. Bright side, you can't get hit! Really good against Duo and really tough bosses.