It looks really good but there is a little bit of a problem with your data structure. Technically rarity, flavor text, number, and artist can all be unique per set. You would be better off including that information in your "prints" sub-array with each set.
The big one with rarity is Serra Angel and Hypnotic Spectre as both of those have switched between uncommon and rare, as well as the original Planeswalkers were just rare in Lorwyn, not mythic as that rarity did not exist at that time.
Another suggestion is to remove the empty fields. It doesn't really matter too much normally, but it could be a slight boost in performance for mobile apps taking advantage of this API as they would essentially be pulling down less data, especially if it is a multi-card search. (100 bytes isn't too much for 1 card, but 100 cards suddenly makes that 9.7kb of extra data that isn't necessary.)
The big one with rarity is Serra Angel and Hypnotic Spectre as both of those have switched between uncommon and rare, as well as the original Planeswalkers were just rare in Lorwyn, not mythic as that rarity did not exist at that time.
Another suggestion is to remove the empty fields. It doesn't really matter too much normally, but it could be a slight boost in performance for mobile apps taking advantage of this API as they would essentially be pulling down less data, especially if it is a multi-card search. (100 bytes isn't too much for 1 card, but 100 cards suddenly makes that 9.7kb of extra data that isn't necessary.)
Looks really great though, very responsive.