You could go for a kind of "gentleman assassin", or I suppose in this case "gentleperson assassin" would be more appropriate - how about a three piece suit of a suitably Victorian or Edwardian cut, with a bowler hat, and then come up with some hidden (but not too hidden) weapons, perhaps a wrist bandolier of throwing knives or some "poison" bottles, holsters with pistols and perhaps more exotic sidearms, either on your belt or shoulder holsters (or both), stuff like that.
The great thing about a suit is that it looks good on men or women, and you could further accessorise depending on whether you want to present a more feminine or masculine side. My missus has recently acquired a nice grey herringbone two piece single breasted suit, so I found her a waistcoat to match and she's got some nice patent leather shoes to go with it, topped off with a trilby. She can either wear it with a shirt and tie or cravat, or with a high necked lace blouse. Unfortunately I don't have any photos of her in it yet or I'd post one to show you, but hopefully you get the idea.
I don't know if you've ever seen the TV show "Miss Fisher's Murder Mysteries" set in 1920's Australia, but one of the characters is Dr Elizabeth "Mac" Macmillan. She's a brilliant character - a highly skilled surgeon who also happens to be a lesbian. She tends to dress in a very masculine style, and I have to say she is one of my favourite characters, partly because she clearly doesn't give a fig about what people think of her "lifestyle choices". In any event, her attire might also provide some inspiration.
I hope this helps
