Dr. Henry Jekyll is a character and antagonist in Once Upon a Time. He is a doctor living in the Land of Untold Stories. He debuts in the twenty-second episode of the fifth season.
He is based on the main titular character of Robert Louis Stevenson's novel Strange Case of Dr. Jekyll and Mr. Hyde.
Before the First Curse
Dr. Jekyll, a scientist living in England, creates a serum that can give a man control over his darker tendencies. He hopes that this serum can grant him acceptance into the academy, which in turn can give him the supplies he needs. He asks Dr. Lydgate, a member of the academy, to come to his lab so he can present the serum to him. Jekyll also has unreciprocated affections for Dr. Lydgate's daughter, Mary, and she helps him prepare for her father's visit. When Lydgate realizes Jekyll's intentions of becoming a member of the academy, he refuses Jekyll's work, saying it is too dangerous. Mary comforts Jekyll before leaving with her father.
Frustrated, Jekyll throws a beaker into the fireplace. Rumplestiltskin soon appears in Jekyll's lab, and finishes the serum for him using magic. Rumple insists that that darkness within the doctor is actually power, not something to fear, and encourages Jekyll to use it to get what he wants: Mary. Jekyll drinks the serum, transforming into a completely different person. Under this guise, he arrives at a party that Dr. Lydgate and Mary are also attending. After seeing his new form in the mirror, he remarks that he does not feel or look like Jekyll. Rumplestiltskin explains to him that he is not Jekyll, he is merely a part of the doctor that Jekyll had wished to hide. this in turn, gives Rumple an idea for a name for the man: Mr. Hyde.
Later, Jekyll awakens with no memory of what happened during his transformation. Rumplestiltskin suggests he look in his pocket, and Jekyll finds an academy pendant, meaning that he was finally accepted, thanks to his other half. However, Rumple points out that Jekyll did not get Mary, causing the doctor to take the serum a second time. He awakens later, this time in bed with Mary, and has no recollection of what happened. Mary is shocked to see him, and Jekyll realizes that she had fallen in love with Hyde, and becomes angry when she tells him that she could never love him. This results in Jekyll accidentaly shoving her out the window, where she falls to her death. Fearful of being caught, he drinks the serum once more, leaving Hyde as the only possible culprit in her murder.
Later, back at Jekyll's lab, Rumple deems Hyde a failed experiment because of his love for Mary, and forces him, and Jekyll by extension, into the Land of Untold Stories.
After the Third Curse
In the Land of Untold Stories, Dr. Jekyll was a doctor who worked in a mental asylum, and his alter ego Mr. Hyde is and takes over the asylum as the warden, forcing Jekyll to become a prisoner. To keep Jekyll from escaping, Hyde makes an orderly at the asylum named Poole keep watch over his other half.
Secretly, Jekyll begins growing a purple flower on the grounds of the asylum that will serve as the final ingredient in another serum that will permanently separate the doctor and Hyde into two separate people. Unbeknownst to Jekyll, Mary Margaret, David, Captain Hook and Zelena are magically transported to his land after The Apprentice's wand malfunctions. As Jekyll is collecting one of the flowers, the four approach him asking for his help. They ask him for magic so they can repair the wand, which they need to return back to their home in Storybrooke. Jekyll declines to help them, and tries to get them to leave, as he is afraid that the warden will discover them. Before they can leave, Poole tasers the four, knocking them unconscious, and imprisons them in a large cage in the asylum.
Jekyll later visits the four in the cage and agrees to help them only if they agree to promise to take him back to Storybrooke with them, explaining that he was made a prisoner after the warden took control. He takes the wand from Zelena, saying he will fix it in the lab as he is unable to take things from it without the warden noticing.
He successfully manages to fix the wand in the lab, but is caught by Poole. Jekyll lies to Poole as to why he is in the lab, saying that Hyde wanted him to fix the wand, calling it a surgical instrument. Poole attacks Jekyll, knocking the wand out of his hands and forces him to drink the serum transforming him into Hyde.
Later, Jekyll awakens as himself again, trapped in a straightjacket. He manages to free himself while Poole is asleep and steals the keys to the cage to release Snow and the others. He takes the four of them to his old home, and while there, adds the flower to another serum that would be used to defeat the warden, without telling them that he and the warden are the same person. However, Poole manages to find them, and forces Jekyll to drink the serum, causing him to transform into Hyde. Snow figures out who Jekyll really is and she and the rest of the group escape as Jekyll transforms. When he becomes Hyde, he injects the new serum into himself, giving himself and his evil persona separate bodies. When Hyde tries to choke Jekyll, Hook returns and knocks Hyde over. Zelena knocks out Poole, and Snow zaps Hyde unconscious with Poole's taser. Before leaving, Jekyll takes what remains of the new serum. Jekyll and his new allies escape into the city where they are eventually found and cornered by Hyde in an alley. Everything is brought to a halt when pennies begin to rain down from the sky, and a portal back to the Land Without Magic opens. Everyone but Hyde jumps into the portal, arriving in a fountain in New York City. Before the group returns to Storybrooke, Jekyll gives the new serum to them as a thank you for saving him. When everyone returns to Storybrooke, David brings Jekyll to Granny's diner.