Social Links
The nature of humanity is such that the relationships we have with others can have a profound effect on our actions and thoughts. We might be able to exist alone, but it is the bonds forged with others, for good or for ill, that allows the true strength of the heart to be made manifest.
When you interact with others, your bonds may strengthen with them... and through them, the archetype that they are a part of. As your bonds within that Arcana strengthen, your will gain access to new Personas of that Arcana. Additionally, some Personas may become available when your overal Social Link progression reaches a certain point. Who knows what depths the heart and mind might truly hold?
Some Arcana might take more effort to advance than others, however. Additionally, dramatic events may result in the advancement of links with others based on your shared experiences. Who can tell what form those experiences might take?