Satan was created first, before there were any angels or demons. He was created to be God's earpiece, never God's equal. Evil is not the opposite of Good, that's not how the balance works. Hatred is not the opposite of Love - apathy is.

The yin and yang demons make up the triumvirate of Hell. It keeps them arguing and stops them being a threat to Heaven.

You think of balance in terms of Yin and Yang, light and dark, good and evil, etc. You focus on the polarity.
Look at a seesaw. Look at scales. Without the bit in the middle there would be no way to have balance or polarity.

There is more behind these equations that people think.

You think if you know "1" then you know "2" because 1 + 1 = 2.
But do you know what "+" is?
Do you know what "=" is?