Board Thread:Game Discussion/@comment-9645402-20130706124427/@comment-10101308-20130708061001

Interestingly, it appears that the number of "filled" stars varies, so not only the rarity matters, but also how much you can promote them (so Kiwi's Scarlett can only be promoted once more)

For anyone that didn't know, there's a matching Wiki for Battleground - keeps this login too, which is nice.