A lot of the fanfiction focuses on the power struggle. Jeff has his own dark power, and sometimes his sister also has some sort of power, and there is a struggle between them in terms of who is more powerful and how that power is used. Also, the theme of protection is there. The sister may try to protect others from Jeff or protect Jeff from himself.
In many 'Jeff the Killer Baby Sister' fanfictions, the theme of discovery is prevalent. The sister discovers Jeff's true nature, discovers her own place in this strange situation, and discovers ways to either change things or come to terms with them. It's a complex web of themes that make these fanfictions so engaging to read.
One common theme is family. It often explores the relationship between Jeff and his sister in the context of his transformation into a killer. Another theme could be protection, like how the sister might try to protect Jeff or vice versa. Also, there's the theme of horror and how she copes with the terrifying world Jeff has become a part of.
Another theme is often about the psychological aspect. How the female Jeff's mind works, what drives her to kill. It might also explore her backstory more deeply, like if she had some sort of trauma in her past that led her to become a killer. Fanfictions often go into detail about her inner thoughts and emotions, which makes it different from the original male Jeff the Killer concept.
Another theme is the struggle between Jeff's dark nature and his newfound feelings. His love for someone might conflict with his urge to kill. This creates a lot of internal and external drama in the stories. Like in a lot of fanfictions, Jeff has to fight his own instincts to protect the person he loves.
One common theme is the struggle between good and evil. Jane is often portrayed as more of the 'good' side trying to stop Jeff who is seen as the 'evil' one. So there's a constant battle of morals.
Often, there are the victims or potential victims of Jeff. They might be ordinary people in the neighborhood who start to notice strange things going on and suspect Jeff. Another common character could be a detective or a police officer who is trying to solve the mystery of the killings and might eventually cross paths with the little sister.
A rather unique 'Jeff the Killer Baby Sister' fanfiction I've read is about her being in a parallel world where Jeff never became a killer. She experiences a completely different life with her brother, but then things start to change when elements from their real world start to seep into this parallel world. It's a long and detailed fanfiction that really dives deep into their relationship.
One common theme is the exploration of the 'outsider' status. Since both are figures from horror, they are outcasts in a sense. Fanfictions often use their relationship to show how two beings that don't fit into the normal world can find connection with each other.
One common theme is the battle between good and evil within their relationship. Since Jeff is a killer and Jane is often portrayed as more of an anti - hero or hero figure, their interactions can be seen as a struggle between these two forces.
One common theme is fear. Since Jeff the killer is a horror character, the fanfictions often play on the reader's fear of him. Another theme is the idea of an unlikely connection. For example, the reader might be drawn to Jeff's dark side in some way.
One common theme could be the exploration of their dark and twisted natures. Since both characters are from the horror genre, their fanfictions might focus on their evil deeds and how they interact in a world full of chaos.