Alternate Stage 5
Defeat Death in Stage 5, then go to Alternate Stage 4 and defeat Camilla.
Dracula X: Rondo Of Blood: Play as Maria
When the minotaur chases Richter in Stage 2, get the Key subweapon from the candlestick. Go down into the sewers after the minotaur gets blocked by the wall. Use the Key subweapon on the locked door at end of the sewer. This will trigger an intermission sequence showing Maria joining Richter. She can now be selected as a playable character at the main menu.
Original Game
Using Maria in stage two, fall down the first pit when the behemoth is chasing you and beat the rest of the level. You will then be taken to a secret stage three. Eventually, you will come to a point that has two paths. Take the upper path and continue along. You will soon come to a point that has platforms on the top and bottom. Break the vine at the top and continue to jump on the top platforms. Break the tombstone at the end and you can play the original version of Rondo of Blood!
Play as Maria (Rondo of Blood)
In Stage 2, while the Behemoth is chasing Richter, there is a candlestick that contains the "key" subweapon. Make sure to get it, and then proceed down into the sewers after the Behemoth is blocked by the wall. At the very end of the sewer path, there is a locked door. Use the key subweapon and it will unlock, giving you a cinematic scene where Maria joins Richter. She is now selectable as a playable character in the main menu.
Rondo of Blood: Stage Select
To select any stage without the work, enter X-X!V'Q as your name.