She was a succubus who fed on the sexual desires of men and women to gain strength. That was her only purpose. She spent centuries this way before she realized she couldn't take it anymore. She wasn't like her brothers and sisters who were content wasting the rest of eternity in the bed of strangers. She wanted spontaneity, she wanted adventure and meaning. She wanted... to become someone else.
The succubus established a new life. She possessed the corpse of a child, and was adopted, named, and raised in the home of a man who lost everything. He taught her how to be just and kind, how to respect and experience the wonders of mother earth. He just so happened to practice sorcery. His simple lifestyle, his teachings of morality and magic, it all fascinated her. He taught her how to focus her magical energies, to protect and preserve herself and the people around her.
Yet as a demon only some of these teachings truly stayed with her. She learned to use her new abilities to kill, and to defend herself. She used her knowledge of humans to better manipulate them, even practicing her former mannerisms as a succubus. Her demonic nature never truly vanished. She gained a new body, a new life, but doing that or more could never change who she was at heart. Still and all, she never allowed the man, her father, to recognize what she was. A fiend. She cherished him, and he was the only person in her entire existence that she ever loved.
But like all humans, he perished, while Lexa remained in her body. Never aging.
She joined The Veil as a medic, using her sorcery to heal and mend her allies broken bodies.