During battle, players will occasionally have the chance to negotiate with demons.
Demons available for negotiation will have a TALK! button above them. Tap it to negotiate with the demon.
Then, choose conversational options that will make the demon happy, and when Demon Talk is successful, the demon will become your companion.
During Demon Talk, demons may request items or your existing companion demon's HP as tribute. If you wish to recruit the demon as a companion, it will be best to comply to their request as much as possible.