In a franchise as prolific as Castlevania, I've always liked the idea that each character could be granted a specific, representative piece of music as his theme. It always seemed to give them more character, more individuality.
While not all of CV's heroes are assorted with an eponymous tune, many consider, as a convention, that a protagonist's theme is the first track that plays for him in the castle's first area (generally the castle's entrance or its main hallways). A supporting or an enemy character's theme will usually play repeatedly when interacting or battling with him.
So, if we were to take universal character themes for granted -- which might or might not be the case --, here's what I'd imagine a comprehensive list would look like (your job, of course, is to comment and correct):
Belmonts:
Gabriel: Belmont's Theme (?)
Leon: Leon's Theme
Trevor: Beginning
Christopher: Battle of the Holy (?)
Simon: Simon's Theme
Juste: Successor of Fate
Richter: Divine Bloodlines/Bloodlines Bequeathed
Julius: Heart of Fire
Non-Belmont heroes:
Hector: Abandoned Castle (?)
Alucard: Castle Dracula (?)
Shanoa: An Empty Tome
John Morris: Reincarnated Soul
Eric Lecarde: Iron Blue Intention
Jonathan Morris/Charlotte Aulin: Invitation of a Crazed Moon (?)
Soma Cruz: Ruined Castle Corridor/Destined Cruz
Reinhardt Schneider: ??
Cornell: ??
Known Enemy Themes:
Death: Evil's Symphonic Poem
Dracula: Dance of Illusions (among others)
And, if you want to go further:
Supporting character themes (among others):
Lydie: Lydie's Theme
Maxim: Maxim's Theme
Mina: Mina's Theme
Joachim: Melancholy Joachim
Rinaldo: Rinaldo's Cabin
etc, etc, etc... (You'll usually find the character's name in the track title)
Are you as theme-crazed as I am when it comes to Castlevania characters? Comment, discuss, etc.