Bilhah

In the Book of Genesis, Bilhah is a concubine of Jacob, and bears him two sons, Dan and Naphtali. See Genesis 30:3-5, 35:25.

She is the handmaiden of Jacob's favourite wife Rachel, who offers her to Jacob when she is herself apparently unable to bear Jacob a son. Rachel is delighted when Dan and Naphtali are born, as they will count as her heirs too.

Reuben loses his right to a double portion as firstborn when Jacob hears of his incest with Bilhah, see Genesis 35:22, 49:3,4, Deuteronomy 21:17.