Sarah's got daddy issues.
He lives in her head, built her out of fish, and killed millions of people.
But he's really sorry. Honest.
A father that lives in your head wouldn't be so bad if he wasn't the killer of millions. At least it's comforting to know that he didn't murder the fishes used to create your body. Or the seagull. Sarah hides her illegal nanite origins in an effort to build an ordinary life, but the legacy of dad's horrors makes it difficult. Especially when new but familiar zombie-like abominations begin to appear in the city.
Echoes of Erebus is a return to more of the action that the first book in the Lifehack (Lifehack) series had. Where Lifehack was much more about military hardware for combat, Sarah in Echoes of Erebus is a civilian with unique abilities. In effect, she can slow her perception of time, allowing her a lot of time to consider her next move.
This doesn't give her any more speed than her technically efficient muscles can give, but if watched from outside her head, she appears to e able to make lighting fast choices, and adapt in an instant... meanwhile from her point of view, she could watch a few movies stored in her head between one moment to the next.
Echoes of Erebus has none of the romantic sub-plot the other books in the series too. She's far too busy trying to figure herself out, her body's abilities, and her place in the world, while also contending with the growing monster menace.
In a way, the Lifehack series follows the arc of her father, and the threat his nano-technologies pose to the world, and the people affected by it. Sarah is particular has to come to terms with her relationship to him, and what it means to be born of illegal tech.