Man/Woman ?

Old/Young ?

Nationality ?

Social Status (Important at the time )

I would have a young female Lady's maid.

She would know her position'but have a real spark about her and a good sense of herself.

Being a Lady's maid'she would travel (Abroad in Winter/Scotland for the shooting season/London or country estate through the rest of the year )

This would give her plenty of new locations to investigate crimes.

She would glean tidbits of knowledge and clues from overhearing the Upstairs Chatter over afternoon tea or a dinner party. 

And learn scandal and insinuation from her colleagues downstairs (or below stairs)that would help her investigations.

Her Lady would never part with her as she has come to rely upon her discretion and efficiency.

Being a Lady's maid and not in a position to confront a high standing suspect, she would need to turn her evidence over to the relevant investigating Police Inspector'who would either accept this grudgingly or come to see her as an equal in an unfair society.

Who would you create and why ?