Bilhah.

Genesis 29:24
And Laban gave his servant girl Zilpah to his daughter Leah as her maidservant.

Genesis 30:3-8
Then she said, "Here is my maidservant Bilhah. Sleep with her, and she will bear children for me, so that through her I too can build a family." . . .

Genesis 35:22, 25
While Israel was living in that region, Reuben went in and slept with his father's concubine Bilhah, and Israel heard about it. Jacob had twelve sons: . . .

Genesis 37:2
This is the account of Jacob. When Joseph was seventeen years old, he was tending the flock with his brothers, the sons of Bilhah and Zilpah, his father's wives, and he brought their father a bad report about them.