Demons are an integral piece of Shin Megami Tensei IV: Apocalypse, and you’ll want to recruit plenty to use as powerful allies in battle.
To try and recruit a demon in battle, you’ll need to have Nanashi select the “talk” option. This will then bring up a conversation where the demon asks you something, and you need to select the right response. This is the trickiest part of recruiting demons, as each one might ask you different questions and need a different response to be satisfied. There are types of demons that ask the same questions, and as you progress through the game you’ll start to learn the patterns. If you fail in conversation enough times, there’s even a chance the demon will become familiar with you and join you without needing convincing in a battle.
After you’ve convinced a demon in conversations, they’ll often ask for items or other things like a character’s HP as tribute before they join. When you get to this step you can either comply and give them what they want, refuse which often angers them, or cheat in an effort to trick the demon.
There are a ton of different demons to pick from in Apocalypse, just try out different conversation options to see what works for each one.