It may seem strange to list Cherish as a separate character. She is, after all, Marina's alter ego - the Fade-girl she was before she became human. In Arclight, it seemed like Cherish was the closest Rue could come to defining all of the abstracts that comprised how she was addressed by the Fade.
To him, she was cherished, so it's what he called her. For Marina, Cherish's life was the one she tried so hard to remember.
But that was Arclight. In Meridian, things are a little different. There's still only one Marina, but she's got two very different voices in her head. The memories that had belonged to Cherish still exist, but they haven't integrated into the rest of her thoughts. Cherish keeps herself apart, often displaying a will of her own. She can choose to cooperate with Marina, or - more often - antagonize her.
Marina's still trying to carve a life out for herself, while Cherish just wants to go home. It's not really a spoiler, as the dynamic exists from the first few pages, but Marina's got more than an external fight on her hands. She's also got the one against herself because two consciousnesses can't split the same life, and in her human mind if she allows Cherish a place, then she'll lose control of her own body. So she's left with the conundrum of having to not only fight herself, but also a stranger who knows everything about her.