When it comes to obtaining every Persona in the series, this is going to be hard to do. That said, this is the purpose of the Compendium. It is there to record every monster that you have collected and could possibly get in the game. There are two main ways that you can get a demon and the first is by collecting them in battle. When you enter a new dungeon or area, it is likely that you will find new demons there to collect. Simply battle them to win them over to your side. If you’re in Persona 3 or 4, defeating them will give you a chance to add them to your party via the cards. If you’re playing Persona 5, though, you need to negotiate with them. This requires downing them in battle and then choosing the right words to win them over to your side. Also, you need to have room in your party and a high enough level to capture that Persona. The other way of getting demons is through fusion in the Velvet Room. Some are even exclusive to this method so you will need a combination of both collecting and fusing to get every demon in the game.