Say, has anyone come across this nifty little item yet?
The Shiny Leaf is something very rare that can be found by your partner pokemon from time to time, depending on their nature AND the route you happen to be on. It has no game use except to be attached to the pokemon and seen only on their status screen. You can find up to five per pokemon, and it simply will be impossible for certain pokemon to get five, due to certain issues (usually size).
If you do find it--and it is never a garauntee--an exclamation mark will appear above your pokemon's head and it will do a little dance with a green musical note above its head, and your character immediately attaches it to your pokemon. If you do get one, the liklihood of your pokemon getting more decreases further with each find, making five incredibly difficult to attain.
So what is your reward for obtaining these? If you find one go see Lyra/Ethan in New Bark Town and they'll tell you about Shiny Leaves. Get five, then they'll give you a star for your card and convert the leaves into a leaf crown for your pokemon!
For more information look
here or
here.