for a curio to evolve, it needs level 10 to go to stage 2, and 25 to stage 3, as for ranks (C,B,A) I don't believe it is level based, but item based (there is a token that allows you to raise there rank), rarity does not go up (hence rarity in definition, in this type of game, the rarity would only go down)
 "Curio Quest"){kind=icon}
 "Chibi Knight"){kind=icon}
 "Super Karoshi"){kind=icon}
 "Mastermind: World Conquerer"){kind=icon}