Boscha created Jay to not be outdone by Willow, Luz and Amity.
It's revealed that Jay is a grimwalker
that Boscha created to be her daughter. Knowing Boscha, she most likely did it when she found out that her old frenemies had gotten married and had kids and she didn't want them to outdo her. She even demands that Jay makes their daughters her enemies like she did with their parents (which is why she became their friend instead).