Persona 3 focuses around the day to day life of the Main Character. Unlike most RPGs, there is no default name for him--though this is the standard for MegaTen games in general so as to spread the Silent Character Archetype, in the official manga release he is called Minato Arisato. He is unique from the rest of the the SEES members in that he has the ability to change Personas during battle.
Apart from the Main Character, there are nine other playable characters. Each of these people have only one Persona and are each assigned a member of the arcanum. These characters are slowly introduced as the game progresses. It is not until later that other antagonists are revealed, and they also possess the ability to summon Personas. These people, under the group name Strega, have artificially-induced Personas and require drugs to keep their Personas from killing them.