I have to think on this, but the basic deal is this: there are no female space marines. A female would have to be an outsider. If there were such a role, it would be a survivor of the inquisitor's retinue. If we go the route of 'the inquisitor had an entourage and one of them survived" we can go anywhere from an acolyte of that Inquisitor (something like their apprentice in the investigation) to a sanctioned-psyker (which would make the entire squad extremely uncomfortable) to a Battle Sister of the Adepta Sororitas that survived -- though the latter would be a huge overlap with the Adeptus Astartes and probably bog the Space Marine squad down a bit with redundant abilities. A member of the Adeptus Arbites seconded to the Inquisition as an acolyte has interesting possibilities as they are investigators and the Dark Angels find themselves without a Brother Interrogator-Chaplain to show the way in a situation where one would be handy.
I don't want a full Inquisitor for a number of reasons. Also bear in mind that the Dark Angels might find a good reason down the road to off your character to keep their secrets; you're playing with fire on this one. :) This isn't a warm and fuzzy chapter like the Ultramarines. The Dark Angels do not play well with others --and unlike the Space Wolves, they don't let you know where you stand ahead of time. They put up a good facade of being upright and orthodox Space Marines with their house in order, but there's skeletons in the closet and no one knows that except them.