The general idea for summoning a Persona is the same across all of the games. The requirement for doing so, from what we have heard in other games and forms of Persona media, is that the user must feel fear and like their life is in danger. This allows them to summon their other self, which can then assist in preserving their lives. This is also why they are almost exclusively summoned in battles where the user’s life is literally in danger. However, the actual method of how someone summons a Persona in the games differs with each entry, like how the Evoker is used in Persona 3.