Like most things in my life, Jane happened by accident. I'd originally intended her as nothing more than a foil for Amy. In my original conception of the novel, Jane was going to be the timid, well-behaved one. There was just one problem. She didn't want to be timid. By the time I'd finished writing the third chapter, Jane had developed a completely different character. Scrap timid. Jane was self-contained, level-headed, and entirely in control of any situation she encountered, despite all of Amy's propensities for creating chaos (or maybe because of Amy's propensity for creating chaos).
I've never thought about it this way before (thanks for the question!), but I'd say that Jane's literary forebearers include Sherlock Holmes, Harriet Vane (from the Dorothy Sayers novels), and Kate Ross' Julian Kestrel. She is someone who places intellect over emotion-- in fact, I'd say her one weakness is a tendency to shy away from the messiness of human relationships. She doesn't like what she can't analyze, and romantic relationships absolutely fall into that category (her own, that is; she's quite happy to bend her logical mind to sorting out other peoples' romantic tangles).
The other inspiration for Jane was historical. There were a number of female spies trotting around during the time period, including a French woman who went under the code name La Prime-rose, who was apparently a mistress of disguise, managing to effectively disguise herself as a man, among other deceptions.
