Charles Darwin helps Jacob destroy the Soothing Syrup distillery in Sequence 4 and then gives the Charles Darwin London Stories, in which the twins protect his work and reputation from Templar-backed attacks.
In Unnatural Selection Darwin is trying to get into the factory that makes Starrick's Soothing Syrup. Jacob clears the guards, the two go in together, find the Syrup is made with opium, and burst the distillation tanks to set the place ablaze. Darwin's ledger from the factory points to Lambeth Asylum. Outside the asylum he confronts Richard Owen, and Jacob chases Owen down to get the maker's name: John Elliotson.
Darwin asks the twins to recover a fossil from a colleague in The Berlin Specimen, though the Templars have already killed the man. They look into a strange flower making people delirious in a Westminster park, trace hostile newspaper stories and caricature posters back to a Templar-funded printer, and free Darwin after a corrupt officer arrests him, with Florence Nightingale's help.
While hunting the flower, the twins tell Darwin more about their war with the Templars, including that Elliotson was one of them. He does not press for details and tells them to keep fighting.
Crawford Starrick
Crawford Starrick is the main antagonist of Assassin's Creed Syndicate: Grand Master of the British Templars and the industrialist who controls London in 1868 through his factories, railways, banks, the Blighters gang and a ring of Templar lieutenants. Every major target the Frye twins kill is one of his people, and the main story builds toward a final confrontation with him at Buckingham Palace.
John Elliotson
Dr John Elliotson is the Templar physician behind Starrick's Soothing Syrup, the addictive tonic dulling London's poor. Jacob Frye tracks him down through Sequence 4 with Charles Darwin's help, and he is the target of the memory Overdose at Lambeth Asylum.
Florence Nightingale
Florence Nightingale is stationed at Lambeth Asylum in 1868. In The Lady with the Lamp she treats Clara O'Dea and asks Evie to recover the supplies needed to replace the counterfeit tonics flooding Lambeth after John Elliotson's death.
Richard Owen
Richard Owen is the naturalist and Darwin rival Jacob chases down in On the Origin of Syrup. Rammed through traffic in his own carriage, he gives up the name of the Syrup's creator: Dr John Elliotson of Lambeth Asylum.