Eurus Holmes

"There's no such thing as bad. Good and bad are fairy tales. We have evolved to attach an emotional significance to what is nothing more than the survival strategy of the pack animal. We are conditioned to invest divinity in utility. Good isn't really good, evil isn't really wrong, and bottoms aren't really pretty."

- Eurus Holmes

Eurus Holmes is the main antagonist of the fourth series of the BBC Series Sherlock. She is the sister of Sherlock and Mycroft Holmes.

She is portrayed by Sian Brooke.

Background
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.

At Sherrinford
Eurus caused many problems at the facility, managing to brainwash multiple people in the facility simply by talking to them, even causing one doctor working there to kill himself and his family. However, she was kept alive for how valuable she was at solving crimes. Mycroft even mentions she managed to predict the exact dates of three different terrorist attacks after just one hour on Twitter. In exchange for her help, Mycroft would bring her gifts on Christmas.

One of these gifts was five unsupervised minutes with Jim Moriarty, whom she had taken an interest in due to his interest in Sherlock. Together, the two planned out a scheme to torment Sherlock that would take effect if Moriarty died. Moriarty also recorded several clips of himself speaking for her to use as part of the plan.

Series 4
Once Moriarty died, Eurus set her plan into motion, first by hijacking every TV in Great Britain to broadcast Moriarty saying "I'm back". She would later disguise herself as a redhead and flirts with Watson in an attempt to drive a wedge in his marriage. She disguises herself again as the daughter of Culverton Smith to get Sherlock to investigate him, and again as Watson's therapist.

Eventually, Sherlock and Watson figure out who she is from an offhand line from Mycroft, and, after she attempts to blow them up with a grenade, the three go to visit her. However, the three soon discover that Eurus had gained control of everyone working at Sherrinford, including its governor. Eurus then has Sherlock, Watson and Mycroft locked up in a room so she can begin her "game" with Sherlock.

She provided Sherlock with a gun and had him choose whether to have Mycroft or Watson kill the governor. If one of them didn't kill him, Eurus threatened to kill the governor's wife. Sherlock initially turned to Mycroft, but he refused, not wanting to have any blood on his hands. Sherlock then turned to Watson, but the doctor couldn't do it either. The governor then frantically grabbed the gun from Watson and shot himself. However, this didn't stop Eurus from killing the governor's wife. Eurus mocked Watson, saying that the only thing that came of his morality was that two people died instead of one. Eurus gave Sherlock a minute of talking time with a little girl on a plane she had apparently rigged to crash. Eurus promised Sherlock more time with her if he solved the next parts of her game.

For the next part, Eurus had Sherlock solve a murder case, presenting him with three suspects. Eurus also revealed that the three suspects worked at Sherrinford, and that she had them tied up, ready to drop them into the ocean. When Sherlock solved the case and "condemned" the guilty one, Eurus dropped the two innocent ones instead. She then dropped the guilty one to see if that would feel different, but it didn't.

For the next part, Eurus presented Sherlock with a coffin and had him determine whose it was. However, Mycroft found the coffin's lid, which read "I love you" on it. Eurus then had Sherlock call up Molly Cooper to get her to say "I love you". If Sherlock didn't get her to say it in three minutes, Eurus threatened to blow up Molly's apartment with explosives she had apparently rigged there. However, when Sherlock finally did get her to say it, Eurus revealed that she had lied about the explosives, rendering the whole ordeal useless.

For the next test, Eurus attempted to get Sherlock to kill either Watson or Mycroft with the gun previously used to kill the governor, stating he could only proceed with one of them. However, Sherlock pointed the gun at himself, prompting Eurus to tranquilize all three of them and drag them back to Sherlock's childhood home. Eurus placed Sherlock in a fake room, and John in the well she had previously killed Victor Trevor in. She also revealed to Sherlock the truth about "Redbeard".

However, Eurus' psychosis eventually broke down and Sherlock discovered that the "girl alone on the plane" was actually a representation of her situation, with her being in solitude miles above all others, understanding everything except how to land and get down to the level of everyone else. Sherlock was able to talk Eurus into helping him save Watson before she was taken away by the authorities and put back in Sherrinford. There, she fell into a completely catatonic state. However, during a visit Sherlock managed to get her to communicate by starting to play the violin with her.

Quotes

 * There's no such thing as bad. Good and bad are fairy tales. We have evolved to attach an emotional significance to what is nothing more than the survival strategy of the pack animal. We are conditioned to invest divinity in utility. Good isn't really good, evil isn't really wrong, and bottoms aren't really pretty.
 * Does it really make a difference, killing the innocent instead of the guilty?

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". However, at this point it was assumed that Mycroft was speaking about a third brother and not a sister.
 * The existence of a third Holmes sibling is greatly disputed by Sherlock fans and followers, although one doesn't appear to exist in the canon works by Arthur Cohan Doyle. The prison Eurus is held at is named Sherrinford, a reference to the proposed elder brother of Sherlock and Mycroft Sherrinford Holmes, with the name also acting as a red herring to the identity to the alluded to secret sibling of Sherlock and Mycroft in the series (as noted above).
 * A younger sister of Sherlock Holmes named Enola Holmes exists in the non-canon works The Enola Holmes Mysteries by Nancy Springer.
 * Eurus and Jim Moriarty could be considered the two main antagonists of the series overall due to the impact on Sherlock's life throughout the whole series, as well as his backstory.
 * Eurus' behavior and interaction with Sherlock is clearly based off Hannibal Lecter, which the guards at Sherrinford make reference to themselves and the film The Silence of the Lambs.