You might have to go through back and forth a lot to get them but it's worth it.

Most of them are from the boss rush.

Unlockable - How to Unlock

Blood-Sucker(Sabata only) - Using Sabata, beat Boss Rush in Gold class with the option "All" and get S-rank.
Day-Walker(Django only) - Using Django, beat Boss Rush in Platinum class with the option "All" and get S-rank.
Eclipse-eye(Sabata only) - Complete Epilogue tower from floors 81-90.
Eclipse-googles(Django only) - Complete the Epilogue tower at the 91-100 floors.
Night-Stalker(Sabata only) - Using Sabata, beat Boss Rush in Platinum class with the option "All" and get S-rank.
Soul-Saver(Django only) - Using Django, beat Boss Rush in Gold class with the option "All" and get S-rank.