Slenderman would be an obvious choice. He has a connection to Jeff in the horror lore. Their interaction could range from being allies in a shared goal, like spreading fear, to being enemies as they compete for territory or influence in the horror world.
You could include characters from other horror franchises. For example, Freddy Krueger. The clash between Jeff's more real - world horror style and Freddy's dream - based horror would create a really unique dynamic. They could be in a battle of who can instill more fear, or they might end up working together to torment a group of people in a new and terrifying way.
The main characters are, of course, Jane the Killer and Jeff the Killer. Jane is often depicted as a vengeful figure, while Jeff is a well - known and terrifying killer. Their interactions are the focus of this fanfic.
The protagonist who has the 'adventures' with Jeff is also a main character. This could be an ordinary person who gets caught up in Jeff's world, perhaps a curious teenager who was in the wrong place at the wrong time. Their reactions and attempts to survive or deal with Jeff form a major part of the story.
If we consider a crossover with the Lord of the Rings universe, Aragorn could be a good fit. He's a noble leader and a skilled warrior. His presence could force the Boys to reevaluate their ideas about power and leadership, and he could also be a strong ally against any corrupted power - hungry characters from the Boys' world.
The Terraria Guide could be a great character to include. He has a lot of knowledge about the Terraria world, so in a crossover, he could be the one trying to figure out the new world they're in. For example, if it's a crossover with Terraria and Hollow Knight, he could study the strange bugs and lore of the Hollow Knight world.
If it's a crossover with Teen Wolf, Stiles would be a great character to include. His quick thinking and sarcastic nature would contrast well with the more serious Shadowhunters. He could also provide some unique insights into solving problems, especially when it comes to the werewolf aspects of the Teen Wolf world that the Shadowhunters might not be as familiar with.
One idea could be a crossover with a popular superhero universe. Jeff the Killer could find himself in a city full of superheroes and villains. He might initially be seen as a threat but then has to team up with some heroes to fight a greater evil. Maybe he uses his stealth and killing skills in a different way to help save the day.
You could include characters like Harry Potter. His knowledge of magic from the Hogwarts world would contrast and interact interestingly with Louise's magic from 'zero no tsukaima'. Another good character could be Naruto. His determination and unique ninja skills would add a different dynamic to the story when combined with Saito's swordsmanship.
In a crossover with Halo, Master Chief would be a great addition. His combat skills and strategic thinking could interact well with the large - scale battles in Supreme Commander. He could lead the UNSC forces against the massive armies in the Supreme Commander world.
In a crossover with the Avengers, characters like Iron Man and Captain America would be great. Iron Man could be intrigued by the technology of the lightsabers and try to replicate it. Captain America, on the other hand, could admire the Jedi's code of honor and fight alongside them against a common enemy, perhaps Thanos who is also after some powerful Force - like artifacts.
One character that would be interesting to include is Batman. His combat skills, detective abilities, and vast array of gadgets would make him a formidable ally against the zombies. Then there's Katniss Everdeen from 'The Hunger Games'. Her archery skills and survival instincts would be very handy. Additionally, Doctor Strange could be a great addition. His magic could be used to create barriers against the zombies or to teleport the characters out of dangerous situations in the 'highschool of the dead' setting.