Powers: As well as flight and super-strength, the Matrix can reform its "limbs" into weapons. At its core, Matrix is disembodied protoplasm that has shaped itself into a human form.
Bio: When Supergirl II merged with the dying Linda Danvers, her alien form was divided over the decision to combine with this person. Ultimately, the two did merge but the part that wasn't used to fill in the holes washed off.
After a while, the Matrix remnants reformed into a kind of evil Supergirl twin, who has dedicated its life to destroying Linda.