Wild Card

The Wild Card is an ability in the Persona universe. Specifically, the ability appears in Persona 3 and Persona 3: FES, however, if all five current Persona games take place in the same universe, it means that the ability has alway existed, only is more rare now. Those who possess the Wild Card can summon and carry multiple Personae and switch them in battle.

Early Persona
In Megami Ibunroku Persona, Persona 2: Innocent Sin and Persona 2: Eternal Punishment, all playable characters were able to switch Personae in combat. However, due to the fact that most non-playable characters were not able to switch Personae supports the theory that the concept of the Wild Card is not exclusive to Persona 3. All those who possessed this ability had encountered Philemon and had access to The Velvet Room, rules that have remains somewhat consistent through the series.

In Persona 3
The protagonist is the only character in Persona 3 to possess the Wild Card. While he never has a direct interaction with Philemon, he is visited by a blue butterfly. Philemon often took the guise of a butterfly in the waking world, so that may have actually been him. Follow that encounter, the protagonist is allowed into The Velvet Room, and thus allowed to create new Persona.

In Persona 3: FES
Aegis is the only character in FES to gain the ability of Wild Card. Like the protagonist before her, she was approached by a blue butterfly before being allowed into The Velvet Room.