I suppose the best way to do this would be to give each Belmont a set of, say, six stats.
Then, use the abilities they use in their games to determine the stat's proficiency.
Lastly, for balance, we make sure that no character can have more points total than any other character.
So let's make that pool... say.... 80 points.
Strength, Defense, Speed, Dexterity, Magic, Magic Defense (normally I include Luck but I won't in this case, because as far as I know, there is no 'lucky belmont')
The average "Hero" would have some 12's, and some 13's (us normal humans would have abysmally smaller stats since we can't use magic, can't superjump, etc, and don't have knowledge of martial arts... though a bodybuilder may rank above a "Hero" in strength alone, and an olympic runner may trump a "Hero" in speed or dexterity).
Strength: affects hitting damage
Can be determined by observing the games and seeing how quickly some common foes fall. You can probably also look at games like Judgment to see knockdown moves vs. combo moves.
Defense: affects taking hits
Can be determined by observing the game's artwork to see who has strong armor on, and who has abilities that can boost their defense.
Speed: affects movement speed
Can be determined by observing the games and looking for obvious movements that increase speed. Most early games will have normal (12-13) speed heroes, but someone like Grant Danesti qualifies for higher speed due to his faster walking. In later games, any hero that can both walk and run gets a speed bonus (Nathan, even if with a powerup). Heroes that autorun (Jonathan, Soma) get a bonus as well.
Dexterity: related to evasive moves that can be performed
This one is easy. Look at the abilities that characters can do to make themselves more agile. Grant can climb walls, high-jump, hang on ceilings, and attack while doing all of it. Soma Cruz has a backdash move and I think Julius Belmont do as well. Richter gets a backflip and a moonwalk in Rondo, and a Superjump and a few other moves in SotN.
Magic: affects magical/mystical damage
This is determined by looking at the games' heroes and looking at the kind of magical power they wield. If it is minimal, you get only a small bonus (or maybe none at all if you're purely physical like Grant), but you get a large bonus if it's your main forte (someone like Sypha, Carrie, or Juste get a large bonus, while someone with just an ItemCrash gets a normal bonus).
Magic Defense: affects avoiding/absorbing/nullifying spiritual damage of any kind
This is determined by moves (not relics) that allow you to soak up, heal, or nullify damage done by magic. For example, Alucard's "Dark Metamorphosis" allows him to soak up life from the blood of enemies. Sonia Belmont gets her "Burning Mode" which gives her invulnerability. Richter gets a small bonus because of his Rebound Stone ItemCrash, which grants him his invincibility. Using the Rosary doesn't count.
Now remember, the number of points is equal on each character.
So now we can do something like comparing Simon Belmont (kinda an average hero), to someone like Richter Belmont (quicker, more magic-oriented):
Simon Belmont
Strength
-Swings the whip fast, implying strength
-concept artwork has him really strong
-is able to swing himself using his whip, implying upper body strength
-it seems that he can do a lot of things while wearing Plate Mail, though it's not a full-body suit.
Defense
-it seems that he can do a lot of things while wearing Plate Mail, though it's not a full-body suit.
Speed
-average speed (no bonus)
Dexterity
-swinging from the whip requires some amount of dexterity
-climbing on stairs isn't something normal people can do, nor is jumping off of them (we bust our ass if we do that)
-can moonwalk on stairs
Magic
-aside from the subweapons (does it count?) nothing really.
Magic Defense
-he is able to have an entire game (CVII) while mystically cursed. There's something to be said about that. But no in-game innate ability.
So clearly, Simon does not excel at magic, but he would have more strength, defense, and dexterity than normal. Here are the numbers (remember, average is 12-13 and we have to pool 80 total):
STR = 30
DEF = 20
SPD = 12
DEX = 15
MAG = 0
MGD = 3
Now we look at Richter Belmont:
Strength: Blah blah 'strongest belmont by far' is bullshit and we're throwing it out the window. It is just not a feasible thing to use. Let's look at sheer damage in the games and the character artwork. He doesn't 'appear' all that muscular in Rondo of Blood (concept art or in-game damage)... and certainly doesn't appear muscular in Symphony...
Defense: Richter is not wearing any type of armor. He seems to be wearing a monk's tunic in Rondo of Blood, and a Nobleman's Frock Coat in Symphony of the Night...
Speed:
-Richter can RUN! Bonus
Dexterity:
-Richter can backflip.
-Richter can run.
-Richter can superjump
-Richter has a go-through-enemies attack that gives him invincibility
-Richter can slide
-Richter can moonwalk (Rondo)
Magic:
-Richter has ItemCrush by default.
Magic Defense:
-MINUS: Richter got his mind possessed by Shaft. Though Simon was cursed, he was able to play through it. Richter succumbed.
-Richter can somehow breathe underwater in Symphony of the Night.
-Richter has a go-through-enemies attack that gives him invincibility
Here are the stats I've determined for Richter:
STR = 14
DEF = 4
SPD = 18
DEX = 20
MAG = 16
MGD = 8
Not too bad, I think...
Unfortunately, this works more for the whole behind-the-scenes level-up RPG system than it does for visible in-game abilities. Balancing out a game using old and new characters would be very challenging.
You might want to look into something like Serio's Castlevania fighter, which has multiple Belmonts, and see how their gameplay varies. Or look into something like Judgment (where for example Simon's dextrous nature allows him to combo a lot of his whip hits, while Trevor's more armored gameplay focuses on knockdown moves and harder but less frequent hits).