Aether

Aether is the life force that inhabits all matter in the universe to a certain degree, but is mostly prevalent in living organisms, whether formed as aetherial cores or as souls inhabiting the bodies of mortals.

Aetherial Cores
When aether gathers and condenses, it can become an aetherial core. Unlike a soul, the aetherial core does not rely on the body it inhabits to shape itself and grow, allowing for functional immortality at the cost of a fundamentally stagnant and unchanging nature.

Mortal Souls
A mortal soul starts small, but grows as it gathers more aether. In sentient races, it can take on a memory and identity that can persist after the body dies if it refuses to disperse back into the aetherial system. As time goes on, it becomes more difficult to retain a memory and identity as the shifting of the planes erode at the exposed aetherial form. Some souls, unable to let go, anchor themselves to the places and objects they knew in life. In special circumstances, this can create more powerful incorporeal beings who keep some manner of form, but lose their memory and/or identity.

Aetherial Plane
Travelling through the aetherial plane as a creature made of matter (through a spell such as Etherealness, for example) would allow you to pass through most solid matter with the exception of concentrated aether and magic barriers that block aether from passing through. Passing through other creatures is either difficult or impossible by this method, and only creatures of pure aether can pass through another.