Satan, or more properly, HaSatan is an angel who resides in the heavenly realm. HaSatan means the accuser or the adversary. The term Satan or HaSatan is used in three different contexts within the Tanach.1
First, as an enemy in war as seen in I Kings 5:18: But now יהוה my God has given me peace on all sides, without opponents (שָׂטָן) or problems.2
Second, as an accuser before the seat of judgment as seen in Psalm 109:6: Set a wicked man over him, and let an accuser (וְשָׂטָן) stand at his right side.3
Third, as an adversary in the general sense of the term as seen in II Samuel 19:23: But David said, “Is this your business, sons of Tzeruyah, that you should oppose (לְשָׂטָן) me today? Should any Yisraelite be put to death? I certainly know that today I am [again] king over Yisrael!”2
As a proper character, Satan appears only once in the Tanach – in the Book of Job. He is depicted as an angel who mocks the piety of the righteous Job.4
Job 1:6: One day, the angels presented themselves before God, and Satan (הַשָּׂטָן) also came with them.3
From the dialogue in the opening chapter of the Book of Job we see that HaSatan is a member of the angelic hosts of the abode of God and has no independent power.
Job 1:7, 12: God said to Satan (הַשָּׂטָן), “Where are you coming from?” Satan (הַשָּׂטָן) answered God, “From going roaming the earth and traversing it.” … God said to Satan (הַשָּׂטָן), “Here, you have control over everything he owns; only do not extend your hand against him personally.” So Satan (הַשָּׂטָן) went out from the presence of God.3
From this exchange we see that HaSatan is an angel who watches over the activities of humanity, searching for mankind’s sins and then appearing as their accuser to God. HaSatan is not considered an opponent to God as Christianity teaches. Monotheistic teachings are no more disturbed by the existence of HaSatan than by the presence of other beings that go before God. This view is shown in Zechariah 3:1-2 where HaSatan is described as the adversary of the high priest Joshua.1