There is a glitch in the game that lets you play as Maria only. However, note that she can not beat the game by herself since she does not have a super jump skill.
To unlock a Maria-only mode, follow these steps. You must have both Richter and Old Axe Armor modes unlocked for the glitch to work:

1. Resume your completed Jonathan/Charlotte mode game.
2. Switch the main character to Charlotte.
3. Save the game. Restart.
4. Choose New Game + with your Jonathan/Charlotte mode game.
5. At the name entry screen, press B.
6. Start a new game and choose Old Axe Armor as the character.
7. Instead of Old Axe Armor, your character will be Maria!