Osiris

Osiris is one of many names of the Cosmic Entity of the Other Plane. A Plane of existance that exists seperate from the Material Plane, and is the home of Souls which have passed on.

Some of his other names include Hades, Mictlantechuli, Ereshkigal, Yama, Anubis etcedra.

In 2143 he appeared to Joel and granted him the Seal of Osiris. Which is a powerful Thamaturgy, made of a billion souls formed into an insignia. It granted Jon the power to slay the Avatars which ravaged the world.

He did watch over Jon and Garfield before Joel's birth. Using Odie as a conduit of his senses. Letting the Dog remain an individual, while still allowing him to see the two much closer.

This explains Garfield's inexplicable hatred of Odie. Which is beyond the hate he had for any other. He himself did not even know.

Avatars
Osiris has multiple Avatars in which it manifests, personifications of the abstracta that make up its being.

Death
The Avatar of it's domain of Death, the most common manifestation being that of a cloaked skeleton holding a scythe.