It can be a Homebrew or an actualy system, but I was wondering of there was a system that emulates the mechanics of the Shin Megami Tensei/Persona series?
If you are not familiar with the basic premise of either in terms of their core gameplay, basically the world that the player is in has demons on the loose, but instead of fighting the demons as well, you are also able to talk to them and be able to possibly convince said demons to be your minions and combine them to make stronger demons (or in Persona's case, make different Personas from combining two different demons with their spell cards). While the two have varying methods of going about it, the concept is pretty much the same.
To understand it a little more, here are some links to the Wiki's to understand a bit better for
Shin Megami Tensei and
The Persona Series