Lorna Walker is a teenager.
That's it. A teenager, aged 16 at the start of the series. Shy and quite introverted. Living with her Aunt with whom she does not get along that well (but then again, she is a teenager so - big shock!). Her mom abandoned her to go live with a cult waiting for a comet or something and her dad ate his shotgun and so the aunt was the only choice. Other than that, she is a normal teenager.
Well, except for a very bad heart. Something to do with a valve not valving right and she is on the list for a new one but there are a lot of people on the list and not that many hearts so figure the odds. Then the dice came up seven and she got the call and raced to the hospital and her life would never be the same.
In Walker's case, this is especially true because with the heart and the medicines to keep rejection to a minimum also came something a theorist (or conspiracy expert) called "muscle memory". She started to have dreams. Exceptionally real and vivid dreams that she became certain were memories, not imaginings. In these dreams she was a he and he was some sort of secret agent or bad ass fighting other bad asses and winning. Except one time. So then he's dead and she has his heart and she now has a lot of his memories.
She also has a fair amount of his skills. When three boys who had had a few too many tried to get too friendly with her and then became insistent that she go along, she put them in the hospital with no effort at all (except for the broken fingernail). What shy, wall-flower of a girl with a heart condition does that?
And this dedicated vegetarian now appreciates a Big Mac. And instead of a Coke, maybe she would have a couple fingers of single-malt Scotch. Neat, please.
So Walker is not a little freaked out about what was happening. And then the really bad dudes from a group called JPAC show up and want to talk. Well, not talk so much as torture and then kill. That is when life gets really, really wrong.