How do you get the good ending in Castlevania Portrait of Ruin?

Answer 1

You must first find the Sanctuary spell in the pyramid level. Then when you fight Stella and Loretta, do not kill them. Use both characters, with Charlotte as your secondary one. Keep the sisters occupied with Jonathan, while Charlotte casts a full strength Sanctuary - don't let them hit her, or the spell will cancel and she'll have to start all over again. This will remove the sisters' curse and return them to normal.

This opens up some more areas and the last parts of the castle, it's fairly linear from then on, and leads towards the good ending.

Answer 2

You must find the Sanctuary spell for Charlotte in the pyramid level. Then once you fight Stella and Loretta, use both characters, Johnathan to keep the sisters at bay while Charlotte casts a double strength sanctuary. Once successful, the sisters are cured of their vampire curse, unlocking four more portraits, Brauner's Lair, and eventually unsealing the barrier close to where you fought the sisters.