Carl Jung defined the Shadow Self as "that which we think we are not." In the process of becoming All that Is, that which we think we are not must be embraced as an aspect of the Greater Self, and integrated.
You may believe that you are not like your neighbour who does this or that bad thing, but if those negative qualities you judge, were not also a part of you, then they would not trigger your emotions.
You may believe that you are a spiritual person who is not cruel or unjust, but that belief is a blockage that casts a shadow that can blind you to the cruelty within your ego that is hidden. The shadow shows the "Ugly Mirror of All that Is": you are what you judge yourself not to be. You are what you judge others to be.
We all have a shadow, and that shadow is a force that is both creative and destructive. We all have an inner demon who can be the violent eruption of represed anger that makes a crime of passion, or the companion to a passionate life that makes skydiving fun because it is scary.
If the Divine Beloved is the representative of God-dess Light within, the Shadow is the reflection of the dark wall of Karma that separates you from knowing yourself as Light. It is a reflection, a dark mirror of Karma. It can perhaps best be understood, in Shamanic terms, as the Portal Guardian.
The Portal Guardian is a creature of non-dual light, unconditional love manifest. It has no duality of its own, no emotions... it is pure light. However, in order to descend into duality-space where humans live, it must have shadow, and since it has no shades of gray within itself, it often first appears as a negative polarised image of darkest shadow. However, it truly has no darkness of its own, its is simply a reflection of your own fears, your Karma. It reflects your dark side.
It is the Shadow that teaches what is hidden on the dark side of the moon.
The Portal guardian is the boundary between your conscious and unconscious mind. It is the protector who keeps you from being overwhelmed by all of your memories at once, by all of the archetypes and minds of the collective consciousness,
...and it is the Gatekeeper who decides what memories are allowed to surface into your consciousness. Some people have to learn to love the Gatekeeper unconditionally before we find access to the Twin soul of the Divine Beloved.
You know the old expressions, "the Devil made me do it"... this expression is a cop-out line for people who are unwilling to take responsibility for their actions, unwilling to admit to the hidden, inner fears and desires that motivate their actions. There is no big bad guy prince of evil. There is no supreme lord of all darkness. Nothing like that.
What there is, is an inner aspect of your own Divinity, whose job it is to reflect your fears and doubts back at you so you may forgive them. People make their own demons, when they refuse to see love in the nagging voices of conscience and worry. The voice of the inner critic who shows you the fearful beliefs that are the content of your Karma.
Demon comes from the Latin "Daemon", which means "inner self". The Divine Beloved is Love. The Shadow is love, too... but it is unconditional love, tough love that spanks. Unconditional love as energy, not emotion. (Kundalini Teacher)