Who/what is he?
He was chief among the angels and quite powerful. How powerful? Hard to pinpoint precisely but to be the top angel should say something. Especially when you consider the acts carried out by other angels in the Scriptures by comparison.
Regardless, he was the first to fall and then became the source of mankind’s fall. He loves to “masquerade as an angel of light” which is just one of several phrases pointing to his title as the deceiver. This same title points to the fact that he doesn’t take a particular form as was seen in the Garden of Eden. In other words, while he could look like the pointy-horned, pointy-tailed red being that seems to have become popular over time, this is NOT normal nor what he would regularly do it.
It is also popular that Satan is somehow the dark foil to God. However, Scripture is quite clear that Satan is in no way close to being God’s equal. Christianity is not a dualism as the two “sides” are not equal.
God is the creator, Satan a creation.
God is infinite (limitless), Satan finite (limited).
God is omnipresent (present everywhere); Satan is singular and can only be one place at a time. God occupies all of creation but is not bound by it as His existence is separate from it. Satan is an object of creation and occupies a definite space within it.
Satan doesn’t stand a chance against God and one day He will end Satan right alongside sin and death (Rev. 20).