Mary Watson (maiden name) Morstan
Mary first appeared in the second Sherlock Holmes novel "The Sign Of Four". She was assisted by Sherlock and Watson and afterwards was married to John Watson and went from Mary Morstan to Mary Watson. She has appeared in the short story "The Man With The Twisted Lip" and is mentioned in other stories by Watson. She disappeared after the return of Sherlock Holmes, its not really explained much in the books but it can be assumed that she died and that's why Watson sold his business and moved back to 221B Baker Street.