Mariam has really always been the Raoul, and she surprisingly turned out more like Raoul in the book than any other Raouls. Obviously she never has a romantic relationship with Lily lol, but "love interest" is not so much what defines a Raoul-type character. Mariam represents for Lily, as Raoul does for Christine, a part of her identity apart from Erik; her childhood thus far; safety; light. She actually matches some of book Raoul's impetuousness and inability to react in a way that doesn't offend Christine and make her want to keep more of her problem to herself. Choosing her, which ultimately Lily does do, is part of why Erik so dangerously retaliates and spirals out of control. Westin just picks up where Mariam leaves off by offering a glimmer of what a decent relationship might be like, and a glimmer of what Lily could do with the part of her personality that was so drawn to Erik, thinking he was the only answer.