Eurus Holmes

Eurus Holmes is the true primary antagonist of the fourth series of the BBC Series Sherlock. She is the sister of Sherlock and Mycroft Holmes and is played by Sian Brooke.

Biography
Eurus is the third child of Mr and Mrs Holmes. When she was a child she murdered her brother Sherlock's best friend Victor Trevor by locking him in a well and allowing him to drown, and refused to give up his location. The situation traumatizes Sherlock to the point where he blots it out and "rewrites" his memories, and imagines his human friend as a dog called "Redbeard", whom Sherlock believes was put down.

After she burns down the Holmes family mansion she is imprisoned at the maximum-security psychiatric prison Sherrinford. Mr and Mrs Holmes assume she is missing with only her brother Mycroft (who works for the British government) knowing her true location. Sherlock himself forgets he ever had a sister.

Trivia

 * The existence of a third Holmes sibling was established in the series three episode "His Last Vow", when Mycroft stated he wasn't prone to brotherly compassion, and told someone that they "knew what happened to the other one".