TLDR
How to keep a character's skillset hidden, when it will be key in the story later on? This must be done without magic and as subtly as humanly possible.
I am working on a medieval fantasy story, where there is a basic premise of magic - it has essentially all but gone from the world but a select few can do basic conjuring (nothing crazy, no wands, no masterful spell casting, just basic stuff almost bordering on illusions). I want the answer to not use magic however - she is not one of those people who have that ability, she is not particularly special in this regard.
Now, in my story there are multiple story lines, flashbacks (via the MC), and a multitude of characters. The story line that this concerns directly involves my MC and his "partner", let's call her Jane. The MC essentially recruited this partner, and we (reader and MC) meet Jane at the same time. Jane is to poise as his romantic interest for a job, although these two are not romantically involved whatsoever. She is a strong character in her own right (think Catlyn Stark, Hermonie Granger, Starbuck in the most recent Battlestar Galactica). The MC hires her and brings Jane along for what we could call a heist. He hires her strictly because she gives off an aire of grace, intelligence and poise (add to that she is attractive) - all things he needs her to exploit to accomplish their task.
About Jane
Jane, is foreign to the land in which our story takes place. She, due to her past, keeps her story hidden and doesn't share much, unless she needs to and even then it's usually through action.
She is
- well spoken, and incredibly intelligent
- can read and write (a skill she doesn't tend to hide),
- multi-lingual (keeps hidden, but speaks both the old tongue, her language, and the common language of the region) as well as having no tangible accent (so even being foreign is hidden for the most part to the undiscerning eye)
- basic field medicine and herbology
- she is also excellent with a dagger, too small for a sword (most people carry some variant of a knightly sword) --- this is a skill she keeps hidden and the skill in question actually.
Question
How can I write about her, letting tidbits of the story go into her background while allowing her to utilize this skill when the required to do so as a result of the story arc? I have gotten far enough in, that I can retcon it to make sense and add details (subtle is fine). I would like her to be a catalyst for change in the story, and killing a particular someone would be the spark that changes things.
I want to avoid a situation where this character essentially steals the spotlight for a moment from our MC via Deus Ex Machina. But I am at a loss as to how to keep this character a tightly guarded mystery, and not have that phenomenon in my story. (I have Checkov's gun covered - she has a dagger she uses incidentally in a non-violent manner).
NOTES
- This is not a duplicate of the this question, as it deals with magic or super powers not a hidden skillset.
- Nor is it a dupe of this question, "How best to avoid the appearance of Deus Ex Machina with established character?" addresses it, kind of. Amadeus' answer briefly touches on it - but doesn't go into detail. Below is the portion of his answer which made me conclude I should ask my own question.
Now you DO want there to be an element of surprise or suspense, not that the TP was capable of saving them, you want that to be plausible from the beginning. The surprise should be that the reader is led to believe the TP must be dead or imprisoned, and then somehow survived and managed to escape himself, to get there on time to save them.
- I have a feeling that utilizing some form of foreshadowing is the key to this question --- but I can't put my finger on the solution. This question here is where I drew some of my ideas from for other events within the same story.