A Jekyll and Hyde - like separation in a Harry Potter fanfiction might involve a character other than Harry too. For instance, Severus Snape could be a great candidate. He already has this complex nature, being a double agent. The separation could make his two loyalties - to Dumbledore and to the Death Eaters in the original story - become two distinct personalities. One side could be fully dedicated to protecting Harry and Hogwarts, while the other is still drawn to the dark side's power and ideology. This would add a new layer of mystery and intrigue to his character and the overall story.
Another possibility is that a dark wizard has created a curse specifically designed to create this Jekyll and Hyde - style separation. This curse could target a prominent character like Draco Malfoy. His 'Jekyll' side might still be the proud Slytherin trying to uphold his family's name, but the 'Hyde' side could be a more desperate and power - hungry version, willing to do anything to gain an advantage. This would lead to a complex character arc for Draco and also impact his relationships with other characters like Harry, Hermione, and Ron in new and unexpected ways.
By exploring the duality of human nature. In 'Harry Potter', characters like Snape have a complex duality - seeming mean but being ultimately good. In 'Jekyll and Hyde', it's more extreme. Fanfiction could show a character with a magical 'Hyde' side that emerges under certain magical circumstances, similar to how Jekyll's potion worked. This would be an effective combination of the themes.
A possible plot could involve a dark wizard who uses the concept of Jekyll and Hyde to create an army of shape - shifting followers. The heroes in the Harry Potter universe have to find a way to reverse the transformation spell and defeat this new threat. This could involve a lot of research into both magical and Muggle concepts related to split personalities and transformation, adding an interesting layer to the story.
Secrecy is another theme. Jekyll's secret about his transformation into Hyde is a central part of the story. In the fanfiction, Utterson's attempts to either uncover or protect that secret are often explored. How he deals with the knowledge that his friend is hiding something so huge is a major part of the narrative in many of these fanfictions. This can lead to a lot of tension and drama in the relationship between Utterson and Jekyll.
Basically, the tale of Jekyll and Hyde centers on Dr. Jekyll's experiments with a concoction that leads to his transformation into the evil Mr. Hyde. It delves into the darker side of human personality and the consequences of unleashing it.
In Jekyll and Hyde fanfiction with time travel, it could be about Hyde traveling back to prevent his own creation. Maybe he realizes the chaos he has caused and wants to change the past. This would add a whole new layer of complexity to the story, as he has to deal with his own duality in a different time period.