Bilhah.

Genesis 29:29
Laban also gave his servant girl Bilhah to his daughter Rachel 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: . . .

Exodus 1:2
Reuben, Simeon, Levi, and Judah;