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.
Definitely not a true story. FNAF is a work of fiction that takes place in a fictional world with its own set of rules and scenarios. It's all about creating a thrilling and immersive gaming and storytelling experience, not reflecting reality.