Hmmm, alright, I think I've thought up something that would work.
Keera and Matt went to school at around the same time and met a few months before graduating from Hogwarts. Since Matt was a DE, Keera thought him week for playing "Follow the leader" but admired his skill in the Dark Arts and so she kept in contact with him. During the years that Keera seemed to disappear from the face of the earth, Matt thought she had died (or was angry with him or something, you can decide) and so, when she returned he was shocked (or angry or w/e, again, you can decide that part). Soon Keera started talking of overthrowing the Ministry and the last of the DEs and told him of her plans for the world (read app for details if you want to know more about that) and she tried to convince Matt to join her little group. Matt refused and she gave up on him.
I don’t know if that idea is any good or not, but If you think it would work then, well, awesome. Also, I have no idea where Keera would fit on the plot; enemies is a little too much but friends isn't accurate.