She wasn't his fallen angel.
She couldn't ever be.
In truth, he wanted to call her an angel - fallen or otherwise, he wanted to call her pure light and goodness, but there was a darkness inside her that he couldn't always understand. There was an evilness and a demon that could easily take over her soul. And even though she kept a leash on her faults, the truth was that they were there. They existed inside her, and so he could never call her a fallen angel because she was never an angel to begin with.
No, she started out as evil, devoid of any natural innocence inside her.
At first that logic bothered Beast Boy, after all how could anyone not have some sort of inkling to be good and honest? How could anyone not be innocent at one point in their lives? It was a natural part of being human to be innocent at birth…
But she wasn't entirely human, was she?
No, she was born part-demon, half-evil, a being never meant to be more than a portal for her father. Expendable, useless, unwanted, unloved.
But he loved her.
God, he loved her.
Faults and all. Her broken innocence. Her demons. Her fears. He loved every single part of her. It was all part of her, and he lovedher. It didn't matter that she had darkness in her soul - they all did, even Beast Boy. He had ghosts in his past that haunted him at every turn, scars that would never heal, and creatures that crept into his dreams at night to claw at him until his was nothing.
But her demons kept his creatures at bay, and he worshiped them with as much reverence as he could.
Perhaps he could never call her a fallen angel, but he could certainly call her a saved demon.