The Planes and the Dimensions that are that laid out within them are all inhabited by beings of greater power as well as mortals and lesser beings.
Those beings of greater power are all ultimately the Children of the Powers that Be, and the descendants of those Powers, and they operate within the Mortality as servants and actors for the Powers that Be.
They do not get along as well as one might hope, but their conflicts can often benefit us.
The assorted Denizens of the Planes have a somewhat complex relationship to the others. In most cases, this relationship can be described in a single term, the ambiguity of each fully embracing the possible ways in which these beings engage with one another. It should be noted that all the major beings are ultimately the children and grandchildren of the Powers That Be, and that their relationships with those Powers are distinct and separate from how they relate to each other.
In all cases, the Denizens of the Planes can be counted on or found serving assorted Powers in varying capacities, most often acting as servants, messengers, and general support staff for the Powers. Sometimes they are given tasks to handle within the Material such as guarding a potential Ikon or supporting someone of interest to a particular Power.
A single Power may have a Devil, Demon, Wraith, Angel, Valkyrie, Ghast, Ghoul, and so forth all serving them at the same time in the same place. This does not mean they get along, however. The following charts show the most common general reactions of the assorted Denizens to one another.
In the Canticle Of Chicory, it is said that the seven Powers of the World were the first to come to The Source, and that they found the Well of Souls, and not the Everlasting Flame, and so had their thirst quenched and became a part of the world itself, whereas the Powers aside from them came to find only the flame and so were burned by it.