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 alleged infidelity with Bilhah, see Genesis 35:22, 49:3,4, Deuteronomy 21:17.
Bilhah her handmaid to wife: and Jacob went in unto her.
Joseph, being seventeen years old, was feeding the flock with his brethren; and the lad was with the sons of Bilhah, and with the sons of Zilpah, his father’s wives: and Joseph brought unto his father their evil report.
Bilhah, which Laban gave unto Rachel his daughter, and she bare these unto Jacob: all the souls were seven.