Satan is Metaphorical

Isaiah 45:6 King James Version
…I am the Lord, and there is none else.

Isaiah 45:7 King James Version
I form the light, and create darkness: I make peace, and create evil: I the Lord do all these things.

2 Nephi 2:11
For it must needs be, that there is an opposition in all things.

2 Nephi 2:25
Adam fell that men might be; and men are, that they might have joy.

At the beginning of time before creation only God existed, thus all of the material universe had God as its only source. Since all of the material universe is projected directly from God, everyone and everything is an aspect of God himself.

Since life is very much more interesting with others than it is all alone God created others from out of himself.

The key ingredient in the creation of other beings was opposition to his will. If everyone and everything always agreed with God all of the time it would still seem like God was all alone. So as 2 Nephi 2:25 suggests Adam fell that men might be. Disobedience was the seed from which human agency (free will) was created.