The first person mentioned in the Bible as a "Hebrew" is Abraham.
"One who had escaped came and reported this to Abram the Hebrew" (Genesis 14:13).
Is Abraham the first Hebrew? The Hebrew word for a "Hebrew" person is עברי (eevriy) and comes from the root word עבר (ever) which means "to cross over." A Hebrew is "one who has crossed over." The name of one of Abraham's ancestors was עבר (ever - Genesis 11:16, written as Eber in English translations). This name also comes from the same root word making it highly possible that Eber is the father (progenitor) of the eevriy (Hebrew) people.
The Bible is the story of God and his covenant relationship (Hebraicly understood as crossing over from death to life) with an ancestral line beginning with Adam through his descendants Noah, Abraham, Isaac, Jacob and Jacob's descendants who became the "nation of Israel" also known as "the Hebrews." It is possible that a Hebrew was one who had "crossed over" into a covenant relationship with God, which would have begun with Adam. When I refer to the "Ancient Hebrews" I am referring to this ancestral line from Adam to the Nation of Israel.