What Connects Demons is Likeability!
Regardless of whether they share a master/apprentice relationship, all demons have some sort of relationship with each other. They will like some and hate some.
The quantification of how much characters like or hate each other is called "likeability"!
The higher their likeability, the friendlier their conversations will become, and the better they will work together in battle.
How Do You Increase Likeability?
Likeability increases as you fight in the same battles, or heal other units with items or spells.
Team attacks really increase your likeability!
This Is...the Power of Love! (Flonne)
Protect Ally Units!
If you share a high likeability with a character, they might take hits for you!
I'm Just Using Them. (Etna)
Follow Up With a Support Attack!
There is a higher likelihood of team attacks with units that share a high likeability!
Also, if two character are close enough, they might perform a support attack!
The more allies there are, the higher the likelihood of support attacking!