These are two of the four characters that can be unlocked.

Character	How to unlock
Aeon	Beat "True" Story mode with him.
Aeon (Alternate)	Connect to Castlevania Order of Ecclesia or find all the soul keys and play story mode after getting all of them. Play with Shanoa and beat Time Reape.
Death	Beat "True" Story mode with him.
Dracula	Beat "True" Story mode with him.
Shanoa	Beat "True" Story mode with her.
Shanoa (Alternate)	Connect to Castlevania Order of Ecclesia or find all the soul keys and play story mode after getting all of them. Play with Shanoa and beat Time Reape.