Experimental42 on 15 July 2014
They were really unclear at the time, but it seems like it's up to each developer to do what they will. Third Parties may also be able to use first party amiibo data in their games, though not the likenesses obviously, and program perks for people with certain data recorded.
Smash so far looks like the amiibo will grant powers for certain modes and act as ghost data. The ghost data could be a welcome addition to the smash tourney community. If it records your responses to certain situations and makes your ghost respond to similar situations based of %s that could be cool. Could use amiibo data collections to test your skills against carbon copies of the best Smash players from around the world.
