Jessica Thomas was not a Chosen One. She hasn't a single little supernatural power, not one prophecy mentions her name and there's a distinctive lack of zombie-vampires going glittery and watching her sleep.
She was always a restless troublemaker, changing classes like others change socks, getting into fights and testing every boundary available, never managing to keep her interest in any one subject up for long.
Then the apocalypse came along, which was a major bummer, but it did deliver a clear purpose for her life. Splattering zombie-guts all over the nearby scenery is just one of those things you just don't get tired of, you know? The remains of mankind can thus sleep soundly in their beds, knowing she and her gang keep the streets clean and undead-free.
She's probably no less unhinged than her two back-up snipers, but she's cute, funny and got boobs, so no-one notices much.