I'm sorry, please don't take what i say so literally, you can upgrade bronzes to gold, but they won't be as powerful unless you really know how to use them...
The number would resemble the "score" of the fighter (Don't know how to call it myself, so i usually call it "fighter score", even though it might not be accurrate), it's the average strength of your fighter, meaning that characters with similar scores (for example, 2500 vs 3000) will be pretty balanced during the fight, while a big difference (another example, 1500 vs 2800) won't make the fight impossible, but it'll be pretty hard.
As for rarities, bronze, silver, gold and diamond are different tiers, each with different max level caps (in order, it would be lvl30, lvl40, lvl50 and lvl60).
Bronzes are naturally the weakest tier, meaning they have the lowest stats and their Signature abilities are usually not so good (With a few exceptions), you can upgrade them to silver, gold and all the way up to diamond if you want, however you'll need to know how to use that fighter correctly in order for it to perform well, and when you evolve characters they go back to lvl1, their base stats are higher, but you'll have to get them to max lvl again, however, their skill tree's upgrade price won't change, so you can still use bronze keys to upgrade them regardless of what tier they are.
Silvers are stronger than Bronzes, but again, they're not special except for a few exceptions, they have higher stats and kinda cool SAs (Short for signature abilities), which would make for a better main character, but they still face the same problem, even though they're stronger, they're not as good as gold fighters, so if you want them to perform well you have to know how to use them.
Golds are where the real fun begins, they have high stats and most of them have pretty helpful and importante SAs, they're really expensive to upgrade, but it's really worth it, they perform far better than the already mentioned, with a few exceptions, you can upgrade them to diamond, but they're not the strongest tier yet.
Diamonds are the highest tier in the game, having some of the highest stats and really good SAs regardless of which fighter it is, they cap at lvl60 and they'll be time consuming, but in that time you'll definitely create a bond with them regardless of which fighter they are (Don't spend a lot of real money just on them though, i'm not against it, feel free to do so if you want, in fact the dev team definitely deserves it, but be aware that natural diamonds will lead you to bankruptcy faster than Thanos could ever snap his fingers).
And that's about it, basically what i said, most bronze not good, half silver good, most gold amasing and all diamond perfekt (And that was my poor attempt of a joke), there are no bad fighters, you can master any variant, just look for one that you'd like and try it for a few days, you'll then know if it's suited for you or not, even if you don't like them give them a chance, and maybe you'll find that hidden gem
![Big Grin :D :D](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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