Who was the Yahwist?

The 'Yahwist', or "J Source", is the name given to one of the anonymous sources now known to have contributed to writing the Pentateuch (Genesis, Exodus, Leviticus and Numbers). He is believed to have lived during the ninth century BCE, and represented the early traditions of the southern Hebrew kingdom of Judah and always used the name Yahweh (YHWH) for an anthropomorphic God with human characteristics, very unlike the depiction of God presented by his near-contemporary, the Elohist.