As I don't know the 'Livye Lewis story', I can't say exactly who this Livye Lewis is. It could be an ordinary person with an interesting personal story, or perhaps someone who is known within a certain group.
There's not enough information to really know who Jeff is. He could be anyone - a fictional character, a real person the storyteller knows, or even the storyteller themselves.
Killer Jeff was a modern horror urban legend character from the United States. There were also derivative works such as movies and games. According to different versions of the story, Killer Jeff was a serial killer who had lost his mind. He was a villain protagonist who had the urge to kill to satisfy his urge to kill. He had a smile carved on his face with a knife. He would sneak into the target's house at night and order the victim to go to bed. If the victim did not listen to his orders, he would use a knife to kill the target. There were many different versions and stories about the origin and background of Jeff the Killer. In some versions, Jeff was an ordinary teenager who became violent because he was bullied and eventually became a serial killer. In the game, players needed to find skulls in the dark maze while avoiding the killer Jeff. In general, Killer Jeff was a character full of horror elements and was very popular among horror culture lovers.
A sympathetic view of Jeff in a love story? That's a tough one. Jeff is a symbol of horror and bloodshed. In a love story, the author might try to explore his backstory and find some redeeming qualities, but he has caused so much pain and death. His insanity and love for killing seem to be at the core of his character. So, while we can try to understand him through the lens of a love story, it's still challenging to be fully sympathetic.
Well, one common character is a female love interest. She could be a strong - willed person who is initially scared of Jeff but then gradually falls in love with him. There might also be a sidekick - type character for Jeff, someone who has known him for a while and tries to warn the love interest about Jeff's true nature. And in some fanfictions, there could be a detective - like character who is trying to solve the mystery of Jeff's killings while also getting involved in the love story. This character might be suspicious of the relationship between Jeff and the love interest and tries to break them up for the sake of safety.
It enriches our understanding. Fiction can fill in the gaps of what we don't know for sure. For example, we can't be entirely certain of the personal thoughts and feelings of Lewis and Clark during every moment of their journey. Fiction allows authors to make educated guesses and present those in a story form.