The de facto leader of the pair, Fran sees herself as “the competent one” having to fight a constant battle against Jamie’s goofy behaviour. In truth, however, Fran’s basically as clueless as Jamie and only takes the lead because Jamie looks up to her, which makes Fran feel responsible for them both.
Fran is far from being a cold, hard killer. She does have a moral compass, albeit a skewed one. Whereas Jamie doesn’t think about it much, Fran frets about the implications of their job and has certain lines she won’t cross (e.g. she’ll never kill an animal).
Fran frequently tries to come up with ways to improve her life, finally deciding (halfway through the series finale) that Jamie is the problem. However, Fran ultimately realises that, in fact, as we’ve seen throughout the series, her friendship with Jamie is the best thing in her life. They might wind each other up, but it’s obvious that they do actually enjoy each other’s company and that, when the shit hits the fan, they’ll have each other’s back no matter what.
Jamie is a well-meaning doofus. Unlike Fran, she’s totally lacking in self-doubt. Impulsive and oblivious to consequences, she will often go ahead and do dumb stuff before Fran can stop her. She is utterly content and doesn’t seem aware of how sinister her job is – she’s clearly just delighted to be able to hang out with her best friend all day, every day. While for Fran the van is a convenient refuge from a dysfunctional home life, for Jamie it’s primarily somewhere to have a laugh.
She’s also super affable and could probably be friends with anyone, even someone they’re trying to kill. Constantly distracted, and always ready with a non-sequitur just at the moment when Fran is trying to concentrate, it would never occur to Jamie that Fran might not be in the mood to hear what she’s thinking.
While Jamie lives in her own world, Fran is very much at the centre of it. To Jamie, Fran isn’t just a best friend: she’s her soulmate. There’s no question in Jamie’s mind that she and Fran will be doing this, or something else, forever – it’s all the same to her so long as they’re together.