Eurus Holmes's History
Not to be confused with Enola Holmes, the Holmes sister in the novel series and 2020 film of the same name.
Eurus Holmes is the younger sister of Mycroft and Sherlock Holmes who was completely unknown to Sherlock until her reveal in "The Lying Detective".
She is the person behind Jim Moriarty's "Did You Miss Me?" propaganda.
Biography
Eurus Holmes is the third child and only daughter of Mr Holmes and Mrs Holmes, eight years younger than Mycroft and one year older than Sherlock, making her the middle child. She is referred to as the cleverest one among the Holmes, classified as an era-defining genius. Mycroft first mentioned her as "the other one". She grew up alongside her two brothers in their family home.
One day, while playing with Sherlock, Mycroft and Redbeard, Eurus displays signs of loneliness and so she decides to kill Redbeard, who is later revealed not to be a dog, but Sherlock's best friend Victor Trevor. Eurus refuses to reveal the whereabouts of Victor and expresses her emotions about the situation through replying to Sherlock with her poem. It is seen in "The Final Problem" that Eurus had cast Victor into a nearby well where he was left to die. The assumed loss of Victor traumatizes Sherlock to the point that he 'rewrites' his memories to forget Eurus and Victor.
Eurus is sent to the maximum-security psychiatric prison, Sherrinford, after she burns down Musgrave, the Holmes' family mansion by her uncle Rudy Holmes. Through her gift of manipulation, she takes control of Sherrinford and can leave whenever she likes. She first meets John Watson while she is undercover, as Elizabeth, a lady on the bus, and this meeting later develops into a texting relationship. Soon after John ends the relationship, she assumes a new disguise as John's therapist, Ursa. Her first meeting with her brother Sherlock on screen is when she disguises herself as Culverton Smith's daughter, Faith Smith, seeking help. She reveals herself to John at the end of "The Lying Detective", when he returns to her seeking therapy, holding him at gun point. The audience is left hanging after Eurus pulls the trigger on John, which was revealed as a tranquilizer dart.
In "The Final Problem", Eurus, having already had the whole institution of Sherrinford at her mercy, plays "games" with Mycroft, Sherlock, and John Watson, making Sherlock choose who would have to shoot the governor, due to the fact that the governor's wife would be shot if they did otherwise. In the end, after no one had wanted to get their hands dirty, the governor shoots himself thinking that his wife's life would be spared. Showing more cruelty, she promptly shoots the governors's wife with no care whatsoever.
Afterwards, because of knowing all about Sherlock's personal life, she lies about having placed bombs in Molly Hooper's flat and then commands Sherlock to get her to make her say that she loved him, without telling her that there was anything urgent going on. While Sherlock's time starts to grow short, she plays recorded clips of Moriarty saying things such as tick-tick-tick-tick in hopes that it would stress Sherlock. It is also revealed that the whole ordeal was planned in five minutes conversation with Jim Moriarty. The next challenge passes, with Sherlock trying to choose which of three men to die, all the while having to save a girl stuck on a crashing plane. After being tranquilized and sent to Musgrave, Sherlock has to save the girl on the plane and a slowly drowning John Watson.
He decodes the cleverly planned gravestones and seeks Eurus in her room, where she is having a delusion, identical to the situation of the girl on the plane, being revealed to be the girl on the plane. After being comforted by Sherlock and saving Watson, she is sent back to Sherrinford, where she doesn't seem to make any conversation besides playing duets with Sherlock. By the end of episode, she seems to have become a different person, due to seemingly being able to feel emotion. She shows that all it needed was Sherlock's compassion, and that the experiments were to see things about emotional context.
