Hunter Watson

Hunter Sherlock Watson is Madison Hunter's son as well as John Watson's and the half-brother of Rosamund Mary Watson. His godparents were Greg Lestrade, Molly Hooper, and Sherlock Holmes.

History
Sometime after or before the death of John's wife, and one of Madison's close friends, John and Maddie have a one-night stand that results in the union of their unborn baby. At the end of The Six Thatchers, Maddie realizes that she is  pregnant after taking a pregnancy test.

She manages to keep it a secret until "The Final Problem," and she later gives birth to Hunter. Hunter later lives in Baker Street with his parents, half-sister, and Sherlock.

Mycroft Holmes
Mycroft knew that Madison was pregnant with Hunter, but didn't know who was the father until Maddie revealed to John that the baby was his. He managed to keep it a secret, but also voiced that Maddie could endanger herself, and thus, her baby as well.

Trivia

 * Hunter's initials "HS" are Sherlock's initials, "SH," backwards