Three words: Hackers. Go. Missing.
I'm not talking baby hackers who have no clue what they're doing and slipped up badly – I'm talking top of the ladder ones.
Mallory Park is a hacker who moderates The Forum, a message board where hackers are anonymous, crack down on the dirty secrets of companies, and leak them out to the world anonymously. She's extremely motivated and passionate about her work, and likes to have control – she's independent, wants to do her own thing, and based on her background, it definitely fits her character. Mallory, however...
Just comes out cold. Apathetic. Indifferent. She doesn't really have any clue what to do with herself because hacking IS who she is. She is also extremely brilliant (I would love to have her math skills right now in Calculus...), hates being touched (much like me...), and is as awkward as David 1 and David 2 (one of them is a Physics major and the other is terrible with metaphors).
While I like Mallory, I think Mallory came more out of her shell when she meets Warden. Warden is very cheery, optimistic, and hilarious – he's really just one of those adorable nerds who would love to squish hug. And despite the fact he is introduced as Mallory's online friend from The Forum, I can hear his voice and see his facial expressions leaping off the screen at Mallory.
There's much anticipation and danger (and Warden's humor laced throughout) as Mallory gets closer to finding out about the missing hackers, but in the long run, Echoes really just teaches about online safety in a similar, yet different way compared to other books related to online safety.
But you should really just read it for Warden's humor.