Bilhah

Therefore it was his will that they who were born from this faulty connection, should yet be reckoned among the legitimate children; just as Moses shortly before called Bilhah a wife, who yet might more properly have been called a harlot.

Commentary on Genesis - Volume 2 1509-1564 1996

The rabbis reveal that Leah and Zilpah knew that they were intended to marry Esau, while Rachel and Bilhah were meant for Jacob.

Zilpah: Midrash and Aggadah. 2009

When Rachel marries Jacob, her father Laban gives her a maid, Bilhah (Gen 29: 29; 46: 25), whom she gives to Jacob as a wife (Hebrew ishah) when she finds herself barren (Gen 30: 3 – 7).

Bilhah: Bible. 2009

Another tradition maintains that the goat in the offering of the tribe of Reuben at the dedication of the Tabernacle (Num. 7: 34) was meant to atone for the episode with Bilhah.

Bilhah: Midrash and Aggadah. 2009

Bilhah bears another son to Jacob under this arrangement, with Rachel again naming the child.

Bilhah: Bible. 2009

Jacob favored Rachel and even loved her maidservant Bilhah more than Zilpah, the handmaiden of Leah (Gen. Rabbati, Vayeze, p. 120).

Zilpah: Midrash and Aggadah. 2009

Along with this tradition, there is also an apparently later midrashic tradition that the nation of Israel had six matriarchs: “Six corresponding to the six matriarchs: Sarah, Rebecca, Rachel, Leah, Bilhah and Zilpah” (Pesikta de-Rav Kahana 1: 7, “And the tribal heads approached”; also Song of Songs Rabbah 6: “You are beautiful”; Esther Rabbah 1: 12, “On the throne”).

Matriarchs: A Liturgical and Theological Category. 2009

At this point Rachel is jealous and has her maid Bilhah give birth to two sons; Leah, no longer fertile, responds by having her own maid Zilpah bear two sons (Gen 30: 9 – 13; 35: 26; 46: 18).

Leah: Bible. 2009

The midrash thereby likens the narrative of Elkanah and Hannah to the stories of the Patriarchs and Matriarchs (Sarah and Hagar, Rachel and Bilhah), in which the beloved wife is barren and initiates the taking of an additional wife in order to produce offspring.

Hannah: Midrash and Aggadah. 2009

According to one midrashic tradition, Reuben did not lie only with Bilhah, as is recounted in Gen. 35: 22, but also with Zilpah.

Zilpah: Midrash and Aggadah. 2009