The girl with no family, memory, or speakable name is the main character and narrative voice of Arclight. For Meridian, she's one of two narrators.
Assumed to be human, she was reported found and rescued in the Grey - the buffer zone between the Arclight and the Dark. Her arrival through the Arclight into a kind of chaos they hadn't known in generations, as she was the first survivor brought inside in decades. She gave the humans hope that there was a way to survive - or even fight - the Fade, but also brought them closer to danger, as the Fade weren't letting her go easily.
But as Marina and her tentative ally Tobin unraveled the truth of her origin, it became clear that she wasn't rescued so much as she was kidnapped. Her memories hadn't been suppressed by trauma, but were being blocked by a chemical administered by the Arclight's medical staff on orders of the facility's leader: Honoria Whit.
In fact, she discovered that Marina wasn't even her name. Honoria gave her the name because "the marina" was where she was found and captured while still a Fade.
In Meridian, Marina is now dealing with the long term repercussions of her decision to stay with the humans, rather than return to the Fade. She believed herself incapable of living as a Fade in their territory, but as more of her memories resurface, she's reminded of everything she left behind. And when those memories take a nightmarish turn, her concern is for everything she has to lose - in both the Arclight and the Dark.
Marina will have to find a way to reconcile both sides of her personality if she's going to help her friends and family.