If you want to unlock Maria as a playable character in Rondo of Blood, try this. Head to the second stage, and go through until you get to the area where the Minotaur enemy is chasing Richter. There will be candles for you to whip as you're being chased. Make sure to hit them all, as one of them is holding a special key, which will appear as your sub-weapon. After the Minotaur chase ends, head to the end of the nearby sewer path. There will be a locked door. The Key sub-weapon will open it. Head on through, and Maria will now be a playable character.