Bilhah

Bilhah was a maidservant to Rachel, who was one of Jacob's two wives.

Biography
When Rachel realized her sister Leah bore Jacob many children, while she bore none, gave Bilhah to Jacob, so she could have children through Bilhah, as that was a custom of that time and place. Bilhah bore two sons to Jacob, Dan, and Naphtali. Leah became jealous, so she gave her maidservant, Zilpah as a wife to Jacob. Zilpah bore two sons to Jacob, Gad and Asher.

Later on, Jacob's oldest son, who was born of a different woman, Reuben, committed adultery with Bilhah.