As for rarity, it can be figured out with the level. everyone with level 25 and higher is legend.
Sorting servants by other statistics would be a nice addition. I support this idea.
oh, interesting about the level = rarity, i just tried to level one of the blue charas i was keeping and she turned into legendary lol
but that makes it too easy for us to get legendary slaves, should be like 10/20% chance for any chara we get to be legendary (maybe not as bad as gacha games, but surely not as easy as it is here)