To unlock Maria Reynard as a playable character, do the following...
In Stage 2: "God, grant me strength", you will come across an area (cutscene) where a disembodied beast (Behemoth) breaks through a passage and begins to chase you. Along the path are candles, as usual. One of them contains a key sub item.
Grab the key, and continue onward to the right, avoiding falling into the pits and the sub-levels below. (That leads to an alternate boss, and to Stage 3b)
You'll come to an area that leads downward. Head down the stairs (optional- before heading down the stairs, break the wall to your right to obtain a pot roast health restorative). The sub-level here is filled with mermen.
Continue to the right. You'll see a stairwell leading up, but also a locked door on the right wall. Use the key item (not item crash- it does nothing) when standing right next to the door.
Once the door is opened, enter the room to trigger the cutscene to unlock Maria Renard.