Genesis 25:6 indicates Abraham fathered children with at least two concubines, Hagar and Keturah. In the ancient Hebrew cultural context, a concubine was a wife with all the legal protections and obligations that implies, but she was also a purchased slave with all of the protections and obligations implied by that status. It was a lesser position than a free wife, but how good or bad life might be depended almost entirely on the culture and family. Just like everything else.