Arez was later engaged/married to Cosette after Keira's execution but she pulls the same betrayal stunt on him and Lord Parvis maybe to usurp power from the House of Parvis and the Magician's Tower. Arez uses his last resort with Lord Parvis to turn back time. Lord Parvis chooses to preserve Keira's memories, thinking that if she were to know of her fate, that she could be the one to stop Cosette. Lord Parvis doesn't preserve his own memories because perhaps he doesn't think he can protect Keira from Cosette's scheming and that Keira's best chance at survival would be to know her fate if she doesn't change. I set up that Arez marries a Parvis daughter because otherwise, he has no connection to the Parvis House that would somehow lead to his caster partner to keep Keira's memories.