According to gamefaqs you should be able to play in either mode after beating the game.
From Gamefaqs:
QUOTE
Stella and Loretta mode is unlocked when you get the True Ending, too. The
sisters can fly to any point on the screen at the expense of some MP. Using
the stylus is a large part of this mode for attacking. The sisters don't have
an item menu, so they can't equip new items. However, you can still level up
by defeating enemies. HP and MP Max Ups are also spread throughout the game.
If you defeat Richter and obtain the True Ending, you will unlock Richter and
Maria Mode, who are characters from Symphony of the Night and Dracula X: Rondo
of Blood. Richter and Maria can't gain new items or abilites, but they can
gain HP and MP Max Ups and level up by defeating enemies. Both Richter and
Maria have special abilites. Richter's Combo (Right, Down, Left, Up and Y)
and Maria's Guardian Knuckle (Up, Left, Down, Right and Y) are their ultimate
attacks.
To unlock Axe Armor Mode, defeat 1,000 Old Axe Armor's in Jonathon's quest and
obtain the true ending. Deadly Axe (Right, Down, Left, Up, Right and Y) is
this character's ultimate attack.
There's more secrets listed in the FAQ:
http://db.gamefaqs.com/portable/ds/file/ca...vania_por_b.txtAnd some pretty cool codes and such here:
http://www.gamefaqs.com/portable/ds/code/930294.htmlSo I'm guessing you can select it when you start a new game? Never played it but I'm sure if you've done it right you should just be able to select it. You should check out the faq page because they have some pretty sweet secrets in that game from what I've been reading.
Never played the game but I kinda wanted to look into it a bit more since I saw the topic... I hate the freakin' DS but I may break down and get one eventually strictly for the Castlevania titles I've missed. They look extremely badass.