Mary Morstan Watson
Some Slight Spoilers if you haven't seen the Third Season since nothing was known of her before that.
Mary's real initials are AGRA. These are printed on the flash drive she gives John once her secret is out, In the original story, "The Sign of Four", Mary's father was part of a group who had a treasure from Agra, India
Sherlock's Initial Deductions about Her

Mary Morstan (Amanda Abbington) and Sherlock Holmes (Benedict Cumberbatch)
Complete list of what Sherlock deduces includes: Linguist, only child, short sighted, Romantic, Guardian (either she reads the Guardian or believes herself to be a guardian), disillusioned Lib Dem, liar, cat lover, size 12, clever, appendix scar, secret tattoo, part time nurse, bakes own bread.

The Mary Reveal

More on the Mary Reveal

Masterpiece: All About Maryl

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