Fuse Devils and Spirits
Megaten Gold is useful during fusing. There are 2 sorts of devils you can fuse devils and spirits.
Fusing with spirits is different to fusing with just other devils, as instead of fusing to make a devil of a different family, it just raises the family level of the current devil by one, or brings it down by one. There are 4 spirits that each affects different devil families differently. The first spirit can be obtained really soon, by fusing a Kahoka with the Kodiaks and this creates the Earth spirit.
There are 5 levels of devil emotion which are neutral, 2 above and 2 below that. If your devil does not like you, its stats will be decreased. The amount depends on how much it does not like you. Try to keep them happy.
There are a few things that affect feelings of your devil towards you. And there is a skill that increases feelings of your devil for you pretty much from one end of the spectrum to the other, so it is a good one to use. But you should take a day to recharge. Summoning your devil also increases its feelings slightly. Things like death and fusing decrease how much it likes you though.
Your devils can do special combo attacks that require a sort of rage gauge to be filled before they can be used. The number of gauges you can fill is determined by your comp, and this combo gauge is explained in a tutorial by the guy standing outside Third Home with a Pixie.
The circles indicate that a new devil can be fused with those 2 devils, and a circle with dots indicates a spirit can be fused. There are some types of demons that simply will not fuse with each other possibly because of alignment. Devils of the same type will not fuse, and certain devils will not fuse with each other.
It is not just a matter of alignment because I have had 2 neutral devils that would not fuse for some reason.