Short answers, pulled from the story.
The Hebrew word satan means accuser or adversary. In 1 Samuel 29:4 it describes a human opponent blocking David's path to battle and Numbers 22:22 uses the same term for an angel of the Lord obstructing Balaam's journey.
Early Christian writers connected Genesis 3's Garden of Eden serpent directly to this fallen angel figure by the time of the New Testament. Origen of Alexandria wrote in the third century comparing the morning star Eosphorus-Lucifer with the Devil himself and argued that Helal-Eosphorus-Lucifer fell into the abyss after trying to equate himself with God.
Augustine described two cities distinct from each other like light and darkness where the earthly city contains wicked men and demons led by the Devil while the heavenly city holds righteous people and angels guided by God. He argued evil came first into existence through the free will of the Devil without independent ontological existence.
From the fourth through twelfth centuries Christian ideas combined with European pagan beliefs creating vivid folklore about the devil. Teutonic gods were often considered demons or even forms of the Devil whom pagans worshipped and sacrificed to and everything sacred to foreign deities became perceived as sacred for the Devil and feared by Christians.
The Malleus Maleficarum written in 1486 became popular extensive work on witch-hunting spreading fear throughout Europe. Martin Luther taught that the Devil was real personal and powerful making evil presumptuous will against God word and creation and affirmed reality of witchcraft caused by the Devil denying only claims about witches' flight and metamorphoses regarded as imagination.